Graphics display system with anti-aliased text and graphics feature

ABSTRACT

A graphics integrated circuit chip is used in a set-top box for controlling a television display. The graphics chip processes analog video input, digital video input, a graphics input and an audio input simultaneously. The system may use anti-aliased text and graphics to provide high quality display of graphical elements, or glyphs, which represent an image of a character of text or graphics, on television and other displays. The graphical elements may be superimposed over live video or arbitrary graphics imagery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.provisional patent application No. 60/107,875, filed Nov. 9, 1998 andentitled “Graphics Chip Architecture,” the contents of which are herebyincorporated by reference.

The present application contains subject matter related to the subjectmatter disclosed in U.S. patent applications entitled “Graphics DisplaySystem” (App. Ser. No. 09/437,208), now U.S. Pat. No. 6,570,579, issuedon May 27, 2003, “Graphics Display System with Graphics Window ControlMechanism (App. Ser. No. 09/437,581), “Graphics Display System withColor Look-Up Table Loading Mechanism” (App. Ser. No. 09/437,206), nowU.S. Pat. No. 6,380,945, issued on Apr. 30, 2002, “Graphics DisplaySystem with Line Buffer Control Scheme” (App. Ser. No. 09/437,325), nowU.S. Pat. No. 6,608,630, issued on Aug. 19, 2003, “Graphics DisplaySystem with Window Soft Horizontal Scrolling Mechanism” (App. Ser. No.09/437,580), “Graphics Display System with Window Descriptors” (App.Ser. No. 09/437,716), “Graphics Display System with VideoSynchronization Feature” (App. Ser. No. 09/437,207), “Graphics DisplaySystem with Video Scaler” (App. Ser. No. 09/437,326), “Apparatus andMethod for Blending Graphics and Video Surfaces” (App. Ser. No.09/437,348), “Graphics Display System with Anti-Flutter Filtering andVertical Scaling Feature” (App. Ser. No. 09/437,327), “Graphics DisplaySystem with Unified Memory Architecture” (App. Ser. No. 09/437,209), nowU.S. Pat. No. 6,189,064, issued on Feb. 13, 2001, and “GraphicsAccelerator” (App. Ser. No. 09/437,579), now U.S. Pat. No. 6,501,480,issued on Dec. 31, 2002, all filed Nov. 9, 1999.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits, and moreparticularly to an integrated circuit graphics display system.

BACKGROUND OF THE INVENTION

Graphics display systems are typically used in television controlelectronics, such as set top boxes, integrated digital TVs, and homenetwork computers. Graphics display systems typically include a displayengine that may perform display functions. The display engine is thepart of the graphics display system that receives display pixel datafrom any combination of locally attached video and graphics input ports,processes the data in some way, and produces final display pixels asoutput.

This application includes references to both graphics and video, whichreflects in certain ways the structure of the hardware itself. Thissplit does not, however, imply the existence of any fundamentaldifference between graphics and video, and in fact much of thefunctionality is common to both. Graphics as used herein may includegraphics, text and video.

SUMMARY OF THE INVENTION

The present invention provides a graphics display system that displaysanti-aliased graphical elements. The system filters the graphicalelement with a low pass filter to generate a multi-level value per pixelat an intended final display resolution and uses the multi-level valuesas alpha blend values for the graphical element in a subsequentcompositing stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an integrated circuit graphics displaysystem according to a presently preferred embodiment of the invention;

FIG. 2 is a block diagram of certain functional blocks of the system;

FIG. 3 is a block diagram of an alternate embodiment of the system ofFIG. 2 that incorporates an on-chip I/O bus;

FIG. 4 is a functional block diagram of exemplary video and graphicsdisplay pipelines;

FIG. 5 is a more detailed block diagram of the graphics and videopipelines of the system;

FIG. 6 is a map of an exemplary window descriptor for describinggraphics windows and solid surfaces;

FIG. 7 is a flow diagram of an exemplary process for sorting windowdescriptors in a window controller;

FIG. 8 is a flow diagram of a graphics window control data passingmechanism and a color look-up table loading mechanism;

FIG. 9 is a state diagram of a state machine in a graphics converterthat may be used during processing of header packets;

FIG. 10 is a block diagram of an embodiment of a display engine;

FIG. 11 is a block diagram of an embodiment of a color look-up table(CLUT);

FIG. 12 is a timing diagram of signals that may be used to load a CLUT;

FIG. 13 is a block diagram illustrating exemplary graphics line buffers;

FIG. 14 is a flow diagram of a system for controlling the graphics linebuffers of FIG. 13;

FIG. 15 is a representation of left scrolling using a window softhorizontal scrolling mechanism;

FIG. 16 is a representation of right scrolling using a window softhorizontal scrolling mechanism;

FIG. 17 is a flow diagram illustrating a system that uses graphicselements or glyphs for anti-aliased text and graphics applications;

FIG. 18 is a block diagram of certain functional blocks of a videodecoder for performing video synchronization;

FIG. 19 is a block diagram of an embodiment of a chroma-locked samplerate converter (SRC);

FIG. 20 is a block diagram of an alternate embodiment of thechroma-locked SRC of FIG. 19;

FIG. 21 is a block diagram of an exemplary line-locked SRC;

FIG. 22 is a block diagram of an exemplary time base corrector (TBC);

FIG. 23 is a flow diagram of a process that employs a TBC to synchronizean input video to a display clock;

FIG. 24 is a flow diagram of a process for video scaling in whichdownscaling is performed prior to capture of video in memory andupscaling is performed after reading video data out of memory;

FIG. 25 is a detailed block diagram of components used during videoscaling with signal paths involved in downscaling;

FIG. 26 is a detailed block diagram of components used during videoscaling with signal paths involved in upscaling;

FIG. 27 is a detailed block diagram of components that may be usedduring video scaling with signal paths indicated for both upscaling anddownscaling;

FIG. 28 is a flow diagram of an exemplary process for blending graphicsand video surfaces;

FIG. 29 is a flow diagram of an exemplary process for blending graphicswindows into a combined blended graphics output;

FIG. 30 is a flow diagram of an exemplary process for blending graphics,video and background color;

FIG. 31 is a block diagram of a polyphase filter that performs bothanti-flutter filtering and vertical scaling of graphics windows;

FIG. 32 is a functional block diagram of an exemplary memory servicerequest and handling system with dual memory controllers;

FIG. 33 is a functional block diagram of an implementation of a realtime scheduling system;

FIG. 34 is a timing diagram of an exemplary CPU servicing mechanism thathas been implemented using real time scheduling;

FIG. 35 is a timing diagram that illustrates certain principles ofcritical instant analysis for an implementation of real time scheduling;

FIG. 36 is a flow diagram illustrating servicing of requests accordingto the priority of the task; and

FIG. 37 is a block diagram of a graphics accelerator, which may becoupled to a CPU and a memory controller.

DETAILED-DESCRIPTION OF A PRESENTLY PREFERRED EMBODIMENT

I. Graphics Display System Architecture

Referring to FIG. 1, the graphics display system according to thepresent invention is preferably contained in an integrated circuit 10.The integrated circuit may include inputs 12 for receiving video signals14, a bus 20 for connecting to a CPU 22, a bus 24 for transferring datato and from memory 28, and an output 30 for providing a video outputsignal 32. The system may further include an input 26 for receivingaudio input 34 and an output 27 for providing audio output 36.

The graphic display system accepts video input signals that may includeanalog video signals, digital video signals, or both. The analog signalsmay be, for example, NTSC, PAL and SECAM signals or any otherconventional type of analog signal. The digital signals may be in theform of decoded MPEG signals or other format of digital video. In analternate embodiment, the system includes. an on-chip decoder fordecoding the MPEG or other digital video signals input to the system.Graphics data for display is produced by any suitable graphics librarysoftware, such as Direct Draw marketed by Microsoft Corporation, and isread from the CPU 22 into the memory 28. The video output signals 32 maybe analog signals, such as composite NTSC, PAL, Y/C (S-video), SECAM orother signals that include video and graphics information. In analternate embodiment, the system provides serial digital video output toan on-chip or off-chip serializer that may encrypt the output.

The graphics display system memory 28 is preferably a unifiedsynchronous dynamic random access memory (SDRAM) that is shared by thesystem, the CPU 22 and other peripheral components. In the preferredembodiment the CPU uses the unified memory for its code and data whilethe graphics display system performs all graphics, video and audiofunctions assigned to it by software. The amount of memory and CPUperformance are preferably tunable by the system designer for thedesired mix of performance and memory cost. In the preferred embodiment,a set-top box is implemented with SDRAM that supports both the CPU andgraphics.

Referring to FIG. 2, the graphics display system preferably includes avideo decoder 50, video scaler 52, memory controller 54, windowcontroller 56, display engine 58, video compositor 60, and video encoder62. The system may optionally include a graphics accelerator 64 and anaudio engine 66. The system may display graphics, passthrough video,scaled video or a combination of the different types of video andgraphics. Passthrough video includes digital or analog video that is notcaptured in memory. The passthrough video may be selected from theanalog video or the digital video by a multiplexer. Bypass video, whichmay come into the chip on a separate input, includes analog video thatis digitized off-chip into conventional YUV (luma chroma) format by anysuitable decoder, such as the BT829 decoder, available from BrooktreeCorporation, San Diego, Calif. The YUV format may also be referred to asYCrCb format where Cr and Cb are equivalent to U and V, respectively.

The video decoder (VDEC) 50 preferably digitizes and processes analoginput video to produce internal YUV component signals with separatedluma and chroma components. In an alternate embodiment, the digitizedsignals may be processed in another format, such as RGB. The VDEC 50preferably includes a sample rate converter 70 and a time base corrector72 that together allow the system to receive non-standard video signals,such as signals from a VCR. The time base corrector 72 enables the videoencoder to work in passthrough mode, and corrects digitized analog videoin the time domain to reduce or prevent jitter.

The video scaler 52 may perform both downscaling and upscaling ofdigital video and analog video as needed. In the preferred embodiment,scale factors may be adjusted continuously from a scale factor of muchless than one to a scale factor of four. With both analog and digitalvideo input, either one may be scaled while the other is displayed fullsize at the same time as passthrough video. Any portion of the input maybe the source for video scaling. To conserve memory and bandwidth, thevideo scaler preferably downscales before capturing video frames tomemory, and upscales after reading from memory, but preferably does notperform both upscaling and downscaling at the same time.

The memory controller 54 preferably reads and writes video and graphicsdata to and from memory by using burst accesses with burst lengths thatmay be assigned to each task. The memory is any suitable memory such asSDRAM. In the preferred embodiment, the memory controller includes twosubstantially similar SDRAM controllers, one primarily for the CPU andthe other primarily for the graphics display system, while eithercontroller may be used for any and all of these functions.

The graphics display system preferably processes graphics data usinglogical windows, also referred to as viewports, surfaces, sprites, orcanvasses, that may overlap or cover one another with arbitrary spatialrelationships. Each window is preferably independent of the others. Thewindows may consist of any combination of image content, includinganti-aliased text and graphics, patterns, GIF images, JPEG images, livevideo from MPEG or analog video, three dimensional graphics, cursors orpointers, control panels, menus, tickers, or any other content, all orsome of which may be animated.

Graphics windows are preferably characterized by window descriptors.Window descriptors are data structures that describe one or moreparameters of the graphics window. Window descriptors may include, forexample, image pixel format, pixel color type, alpha blend factor,location on the screen, address in memory, depth order, on the screen,or other parameters. The system preferably supports a wide variety ofpixel formats, including RGB 16, RGB 15, YUV 4:2:2 (ITU-R 601 ), CLUT2,CLUT4, CLUT8 or others. In addition to each window having its own alphablend factor, each pixel in the preferred embodiment has its own alphavalue. In the preferred embodiment, window descriptors are not used forvideo windows. Instead, parameters for video windows, such as memorystart address and window size are stored in registers associated withthe video compositor.

In operation, the window controller 56 preferably manages both the videoand graphics display pipelines. The window controller preferablyaccesses graphics window descriptors in memory through a direct memoryaccess (DMA) engine 76. The window controller may sort the windowdescriptors according to the relative depth of their correspondingwindows on the display. For graphics windows, the window controllerpreferably sends header information to the display engine at thebeginning of each window on each scan line, and sends window headerpackets to the display engine as needed to display a window. For video,the window controller preferably coordinates capture of non-passthroughvideo into memory, and transfer of video between memory and the videocompositor.

The display engine 58 preferably takes graphics information from memoryand processes it for display. The display engine preferably converts thevarious formats of graphics data in the graphics windows into YUVcomponent format, and blends the graphics windows to create blendedgraphics output having a composite alpha value that is based on alphavalues for individual graphics windows, alpha values per pixel, or both.In the preferred embodiment, the display engine transfers the processedgraphics information to memory buffers that are configured as linebuffers. In an alternate embodiment, the buffer may include a framebuffer. In another alternate embodiment, the output of the displayengine is transferred directly to a display or output block withoutbeing transferred to memory buffers.

The video compositor 60 receives one or more types of data, such asblended graphics data, video window data, passthrough video data andbackground color data, and produces a blended video output. The videoencoder 62 encodes the blended video output from the video compositorinto any suitable display format such as composite NTSC, PAL, Y/C(S-video), SECAM or other signals that may include video information,graphics information, or a combination of video and graphicsinformation. In an alternate embodiment, the video encoder converts theblended video output of the video compositor into serial digital videooutput using an on-chip or off chip serializer that may encrypt theoutput.

The graphics accelerator 64 preferably performs graphics operations thatmay require intensive CPU processing, such as operations on threedimensional graphics images. The graphics accelerator may beprogrammable. The audio engine 66 preferably supports applications thatcreate and play audio locally within a set-top box and allow mixing ofthe locally created audio with audio from a digital audio source, suchas MPEG or Dolby, and with digitized analog audio. The audio engine alsopreferably supports applications that capture digitized baseband audiovia an audio capture port and store sounds in memory for later use, orthat store audio to memory for temporary buffering in order to delay theaudio for precise lip-syncing when frame-based video time correction isenabled.

Referring to FIG. 3, in an alternate embodiment of the presentinvention, the graphics display system further includes an I/O bus 74connected between the CPU 22, memory 28 and one or more of a widevariety of peripheral devices, such as flash memory, ROM, MPEG decoders,cable modems or other devices. The on-chip I/O bus 74 of the presentinvention preferably eliminates the need for a separate interfaceconnection, sometimes referred in the art to as a north bridge. The I/Obus preferably provides high speed access and data transfers between theCPU, the memory and the peripheral devices, and may be used to supportthe full complement of devices that may be used in a full featuredset-top box or digital TV. In the preferred embodiment, the I/O bus iscompatible with the 68000 bus definition, including both active DSACKand passive DSACK (e.g., ROM/flash devices), and it supports externalbus masters and retry operations as both master and slave. The buspreferably supports any mix of 32-bit, 16-bit and 8-bit devices, andoperates at a clock rate of 33 MHz. The clock rate is preferablyasynchronous with (not synchronized with) the CPU clock to enableindependent optimization of those subsystems.

Referring to FIG. 4, the graphics display system generally includes agraphics display pipeline 80 and a video display pipeline 82. Thegraphics display pipeline preferably contains functional blocks,including window control block 84, DMA (direct memory access) block 86,FIFO (first-in-first-out memory) block 88, graphics converter block 90,color look up table (CLUT) block 92, graphics blending block 94, staticrandom access memory (SRAM) block 96, and filtering block 98. The systempreferably spatially processes the graphics data independently of thevideo data prior to blending.

In operation, the window control block 84 obtains and stores graphicswindow descriptors from memory and uses the window descriptors tocontrol the operation of the other blocks in the graphics displaypipeline. The windows may be processed in any order. In the preferredembodiment, on each scan line, the system processes windows one at atime from back to front and from the left edge to the right edge of thewindow before proceeding to the next window. In an alternate embodiment,two or more graphics windows may be processed in parallel. In theparallel implementation, it is possible for all of the windows to beprocessed at once, with the entire scan line being processed left toright. Any number of other combinations may also be implemented, such asprocessing a set of windows at a lower level in parallel, left to right,followed by the processing of another set of windows in parallel at ahigher level.

The DMA block 86 retrieves data from memory 110 as needed to constructthe various graphics windows according to addressing informationprovided by the window control block. Once the display of a windowbegins, the DMA block preferably retains any parameters that may beneeded to continue to read required data from memory. Such parametersmay include, for example, the current read address, the address of thestart of the next lines, the number of bytes to read per line, and thepitch. Since the pipeline preferably includes a vertical filter blockfor anti-flutter and scaling purposes, the DMA block preferably accessesa set of adjacent display lines in the same frame, in both fields. Ifthe output of the system is NTSC or other form of interlaced video, theDMA preferably accesses both fields of the interlaced final displayunder certain conditions, such as when the vertical filter and scalingare enabled. In such a case, all lines, not just those from the currentdisplay field, are preferably read from memory and processed duringevery display field. In this embodiment, the effective rate of readingand processing graphics is equivalent to that of a non-interlaceddisplay with a frame rate equal to the field rate of the interlaceddisplay.

The FIFO block 88 temporarily stores data read from the memory 110 bythe DMA block 86, and provides the data on demand to the graphicsconverter block 90. The FIFO may also serve to bridge a boundary betweendifferent clock domains in the event that the memory and DMA operateunder a clock frequency or phase that differs from the graphicsconverter block 90 and the graphics blending block 94. In an alternateembodiment, the FIFO block is not needed. The FIFO block may beunnecessary, for example, if the graphics converter block processes datafrom memory at the rate that it is read from the memory and the memoryand conversion functions are in the same clock domain.

In the preferred embodiment, the graphics converter block 90 takes rawgraphics data from the FIFO block and converts it to YUValpha (YUVa)format. Raw graphics data may include graphics data from memory that hasnot yet been processed by the display engine. One type of YUVa formatthat the system may use includes YUV 4:2:2 (i.e. two U and V samples forevery four Y samples) plus an 8-bit alpha value for every pixel, whichoccupies overall 24 bits per pixel. Another suitable type of YUVa formatincludes YUV 4:4:4 plus the 8-bit alpha value per pixel, which occupies32 bits per pixel. In an alternate embodiment, the graphics convertermay convert the raw graphics data into a different format, such asRGBalpha.

The alpha value included in the YUVa output may depend on a number offactors, including alpha from chroma keying in which a transparent pixelhas an alpha equal to zero, alpha per CLUT entry, alpha from Y (luma),or alpha per window where one alpha value characterizes all of thecontents of a given window.

The graphics converter block 90 preferably accesses the CLUT 92 duringconversion of CLUT formatted raw graphics data. In one embodiment of thepresent invention, there is only one CLUT. In an alternate embodiment,multiple CLUTs are used to process different graphics windows havinggraphics data with different CLUT formats. The CLUT may be rewritten byretrieving new CLUT data via the DMA block when required. in practice,it typically takes longer to rewrite the CLUT than the time available ina horizontal blanking interval, so the system preferably allows onehorizontal line period to change the CLUT. Non-CLUT images may bedisplayed while the CLUT is being changed. The color space of theentries in the CLUT is preferably in YUV but may also be implemented inRGB.

The graphics blending block 94 receives output from the graphicsconverter block 90 and preferably blends one window at. a time along theentire width of one scan line, with the back-most graphics window beingprocessed first. The blending block uses the output from the converterblock to modify the contents of the SRAM 96. The result of each pixelblend operation is a pixel in the SRAM that consists of the weighted sumof the various graphics layers up to and including the present one, andthe appropriate alpha blend value for the video layers, taking intoaccount the graphics layers up to and including the present one.

The SRAM 96 is preferably configured as a set of graphics line buffers,where each line buffer corresponds to a single display line. Theblending of graphics windows is preferably performed one graphics windowat a time on the display line that is currently being composited into aline buffer. Once the display line in a line buffer has been completelycomposited so that all the graphics windows on that display line havebeen blended, the line buffer is made available to the filtering block98.

The filtering block 98 preferably performs both anti-flutter filtering(AFF) and vertical sample rate conversion (SRC) using the same filter.This block takes input from the line buffers and performs finite impulseresponse polyphase filtering on the data. While anti-flutter filteringand vertical axis SRC are done in the vertical axis, there may bedifferent functions, such as horizontal SRC or scaling that areperformed in the horizontal axis. In the preferred embodiment, thefilter takes input from only vertically adjacent pixels at one time. Itmultiplies each input pixel times a specified coefficient, and sums theresult to produce the output. The polyphase action means that thecoefficients, which are samples of an approximately continuous impulseresponse, may be selected from a different fractional-pixel phase of theimpulse response every pixel. In an alternate embodiment, where thefilter performs horizontal scaling, appropriate coefficients areselected for a finite impulse response polyphase filter to perform thehorizontal scaling. In an alternate embodiment, both horizontal andvertical filtering and scaling can be performed.

The video display pipeline 82 may include a FIFO block 100, an SRAMblock 102, and a video scaler 104. The video display pipeline portion ofthe architecture is similar to that of the graphics display pipeline,and it shares some elements with it. In the preferred embodiment, thevideo pipeline supports up to one scaled video window per scan line, onepassthrough video window, and one background color, all of which arelogically behind the set of graphics windows. The order of thesewindows, from back to front, is preferably fixed as background color,then passthrough video, then scaled video.

The video windows are preferably in YUV format, although they may be ineither 4:2:2 or 4:2:0 variants or other variants of YUV, oralternatively in other formats such as RGB. The scaled video window maybe scaled up in both directions by the display engine, with a factorthat can range up to four in the preferred embodiment. Unlike graphics,the system generally does not have to correct for square pixel aspectratio with video. The scaled video window may be alpha blended intopassthrough video and a background color, preferably using a constantalpha value for each video signal.

The FIFO block 100 temporarily stores captured video windows fortransfer to the video scaler 104. The video scaler preferably includes afilter that performs both upscaling and downscaling. The a scalerfunction may be a set of two polyphase SRC functions, one for eachdimension. The vertical SRC may be a four-tap filter with programmablecoefficients in a fashion similar to the vertical filter in the graphicspipeline, and the horizontal filter may use an 8-tap SRC, also withprogrammable coefficients. In an alternate embodiment, a shorterhorizontal filter is used, such as a 4-tap horizontal SRC for the videoupscaler. Since the same filter is preferably used for downscaling, itmay be desirable to use more taps than are strictly needed for upscalingto accommodate low pass filtering for higher quality downscaling.

In the preferred embodiment, the video pipeline uses a separate windowcontroller and DMA. In an alternate embodiment, these elements may beshared. The FIFOs are logically separate but may be implemented in acommon SRAM.

The video compositor block 108 blends the output of the graphics displaypipeline, the video display pipeline, and passthrough video. Thebackground color is preferably blended as the lowest layer on thedisplay, followed by passthrough video, the video window and blendedgraphics. In the preferred embodiment, the video compositor compositeswindows directly to the screen line-by-line at the time the screen isdisplayed, thereby conserving memory and bandwidth. The video compositormay include, but preferably does not include, display frame buffers,double-buffered displays, off-screen bit maps, or blitters.

Referring to FIG. 5, the display engine 58 preferably includes graphicsFIFO 132, graphics converter 134, RGB-to-YUV converter 136,YUV-444-to-YUV422 converter 138 and graphics blender 140. The graphicsFIFO 132 receives raw graphics data from memory through a graphics DMA124 and passes it to the graphics converter 134, which preferablyconverts the raw graphics data into YUV 4:4:4 format or other suitableformat. A window controller 122 controls the transfer of raw graphicsdata from memory to the graphics converter 132. The graphics converterpreferably accesses the RGB-to-YUV converter 136 during conversion ofRGB formatted data and the graphics CLUT 146 during conversion of CLUTformatted data. The RGB-to-YUV converter is preferably a color spaceconverter that converts raw graphics data in RGB space to graphics datain YUV space. The graphics CLUT 146 preferably includes a CLUT 150,which stores pixel values for CLUT-formatted graphics data, and a CLUTcontroller 152, which controls operation of the CLUT.

The YUV444-to-YUV422 converter 138 converts graphics data from YUV 4:4:4format to YUV 4:2:2 format. The term YUV 4:4:4 means, as isconventional, that for every four horizontally adjacent samples, thereare four Y values, four U values, and four V values; the term YUV 4:2:2means, as is conventional, that for every four samples, there are four Yvalues, two U values and two V values. The YUV444-to-YUV422 converter138 is preferably a UV decimator that sub-samples U and V from foursamples per every four samples of Y to two samples per every foursamples of Y.

Graphics data in YUV 4:4:4 format and YUV 4:2:2 format preferably alsoincludes four alpha values for every four samples. Graphics data in YUV4:4:4 format with four alpha values for every four samples may bereferred to as being in aYUV 4:4:4:4 format; graphics data in YUV 4:2:2format with four alpha values for every four samples may be referred toas being in aYUV 4:4:2:2 format.

The YUV444-to-YUV422 converter may also perform low-pass filtering of UVand alpha. For example, if the graphics data with YUV 4:4:4 format hashigher than desired frequency content, a low pass filter in theYUV444-to-YUV422 converter may be turned on to filter out high frequencycomponents in the U and V signals, and to perform matched filtering ofthe alpha values.

The graphics blender 140 blends the YUV 4:2:2 signals together,preferably one line at a time using alpha blending, to create a singleline of graphics from all of the graphics windows on the current displayline. The filter 170 preferably includes a single 4-tap verticalpolyphase graphics filter 172, and a vertical coefficient memory 174.The graphics filter may perform both anti-flutter filtering and verticalscaling. The filter preferably receives graphics data from the displayengine through a set of seven line buffers 59, where four of the sevenline buffers preferably provide data to the taps of the graphics filterat any given time.

In the preferred embodiment, the system may receive video input thatincludes one decoded MPEG video in ITU-R 656 format and one analog videosignal. The ITU-R 656 decoder 160 processes the decoded MPEG video toextract timing and data information. In one embodiment, an on-chip videodecoder (VDEC) 50 converts the analog video signal to a digitized videosignal. In an alternate embodiment, an external VDEC such as theBrooktree BT 829 decoder converts the analog video into digitized analogvideo and provides the digitized video to the system as bypass video130.

Analog video or MPEG video may be provided to the video compositor aspassthrough video. Alternatively, either type of video may be capturedinto memory and provided to the video compositor as a scaled videowindow. The digitized analog video signals preferably have a pixelsample rate of 13.5 MHz, contain a 16 bit data stream in YUV 4:2:2format, and include timing signals such as top field and vertical syncsignals.

The VDEC 50 includes a time base corrector (TBC) 72 comprising a TBCcontroller 164 and a FIFO 166. To provide passthrough video that issynchronized to a display clock preferably without using a frame buffer,the digitized analog video is corrected in the time domain in the TBC 72before being blended with other graphics and video sources. During timebase correction, the video input which runs nominally at 13.5 MHZ issynchronized with the display clock which runs nominally at 13.5 MHZ atthe output; these two frequencies that are both nominally 13.5 MHz arenot necessarily exactly the same frequency. In the TBC, the video outputis preferably offset from the video input by a half scan line per field.

A capture FIFO 158 and a capture DMA 154 preferably capture thedigitized analog video signals and MPEG video. The SDRAM controller 126provides captured video frames to the external SDRAM. A video DMA 144transfers the captured video frames to a video FIFO 148 from theexternal SDRAM.

The digitized analog video signals and MPEG video are preferably scaleddown to less than 100% prior to being captured and are scaled up to morethan 100% after being captured. The video scaler 52 is shared by bothupscale and downscale operations. The video scaler preferably includes amultiplexer 176, a set of line buffers 178, a horizontal and verticalcoefficient memory 180 and a scaler engine 182. The scaler engine 182preferably includes a set of two polyphase filters, one for each ofhorizontal and vertical dimensions.

The vertical filter preferably includes a four-tap filter withprogrammable filter coefficients. The horizontal filter preferablyincludes an eight-tap filter with programmable filter coefficients. Inthe preferred embodiment, three line buffers 178 supply video signals tothe scaler engine 182. The three line buffers 178 preferably are 720×16two port SRAM. For vertical filtering, the three line buffers 178 mayprovide video signals to three of the four taps of the four-tap verticalfilter while the video input provides the video signal directly to thefourth tap. For horizontal filtering, a shift register having eightcells in series may be used to provide inputs to the eight taps of thehorizontal polyphase filter, each cell providing an input to one of theeight taps.

For downscaling, the multiplexer 168 preferably provides a video signalto the video scaler prior to capture. For upscaling, the video FIFO 148provides a video signal to the video scaler after capture. Since thevideo scaler 52 is shared between downscaling and upscaling filtering,downscaling and upscaling operations are not performed at the same timein this particular embodiment.

In the preferred embodiment, the video compositor 60 blends signals fromup to four different sources, which may include blended graphics fromthe filter 170, video from a video FIFO 148, passthrough video from amultiplexer 168, and background color from a background color module184. Alternatively, various numbers of signals may be composited,including, for example, two or more video windows. The video compositorpreferably provides final output signal to the data size converter 190,which serializes the 16-bit word sample into an 8-bit word sample attwice the clock frequency, and provides the 8-bit word sample to thevideo encoder 62.

The video encoder 62 encodes the provided YUV 4:2:2 video data andoutputs it as an output of the graphics display system in any desiredanalog or digital format.

II. Window Descriptor and Solid Surface Description

Often in the creation of graphics displays, the artist or applicationdeveloper has a need to include rectangular objects on the screen, withthe objects having a solid color and a uniform alpha blend factor (alphavalue). These regions (or objects) may be rendered with other displayedobjects on top of them or beneath them. In conventional graphicsdevices, such solid color objects are rendered using the number ofdistinct pixels required to fill the region. It may be advantageous interms of memory size and memory bandwidth to render such objects on thedisplay directly, without expending the memory size or bandwidthrequired in conventional approaches.

In the preferred embodiment, video and graphics are displayed on regionsreferred to as windows. Each window is preferably a rectangular area ofscreen bounded by starting and ending display lines and starting andending pixels on each display line. Raw graphics data to be processedand displayed on a screen preferably resides in the external memory. Inthe preferred embodiment, a display engine converts raw graphics datainto a pixel map with a format that is suitable for display.

In one embodiment of the present invention, the display engineimplements graphics windows of many types directly in hardware. Each ofthe graphics windows on the screen has its own value of variousparameters, such as location on the screen, starting address in memory,depth order on the screen, pixel color type, etc. The graphics windowsmay be displayed such that they may overlap or cover each other, witharbitrary spatial relationships.

In the preferred embodiment, a data structure called a window descriptorcontains parameters that describe and control each graphics window. Thewindow descriptors are preferably data structures for representinggraphics images arranged in logical surfaces, or windows, for display.Each data structure preferably includes a field indicating the relativedepth of the logical surface on the display, a field indicating thealpha value for the graphics in the surface, a field indicating thelocation of the logical surface on the display, and a field indicatingthe location in memory where graphics image data for the logical surfaceis stored.

All of the elements that make up any given graphics display screen arepreferably specified by combining all of the window descriptors of thegraphics windows that make up the screen into a window descriptor list.At every display field time or a frame time, the display engineconstructs the display image from the current window descriptor list.The display engine composites all of the graphics windows in the currentwindow descriptor list into a complete screen image in accordance withthe parameters in the window descriptors and the raw graphics dataassociated with the graphics windows.

With the introduction of window descriptors and real-time composition ofgraphics windows, a graphics window with a solid color and fixedtranslucency may be described entirely in a window descriptor havingappropriate parameters. These parameters describe the color and thetranslucency (alpha) just as if it were a normal graphics window. Theonly difference is that there is no pixel map associated with thiswindow descriptor. The display engine generates a pixel map accordinglyand performs the blending in real time when the graphics window is to bedisplayed.

For example, a window consisting of a rectangular object having aconstant color and a constant alpha value may be created on a screen byincluding a window descriptor in the window descriptor list. In thiscase, the window descriptor indicates the color and the alpha value ofthe window, and a null pixel format, i.e., no pixel values are to beread from memory. Other parameters indicate the window size and locationon the screen, allowing the creation of solid color windows with anysize and location. Thus, in the preferred embodiment, no pixel map isrequired, memory bandwidth requirements are reduced-and a window of anysize may be displayed.

Another type of graphics window that the window descriptors. preferablydescribe is an alpha-only type window. The alpha-only type windowspreferably use a constant color and preferably have graphics data with2, 4 or 8 bits per pixel. For example, an alpha-4 format may be analpha-only format used in one of the alpha-only type windows. Thealpha-4 format specifies the alpha-only type window with alpha blendvalues having four bits per pixel. The alpha-only type window may beparticularly useful for displaying anti-aliased text.

A window controller preferably controls transfer of graphics displayinformation in the window descriptors to the display engine. In oneembodiment, the window controller has internal memory to store eightwindow descriptors. In other embodiments, the window controller may havememory allocated to store more or less window descriptors. The windowcontroller preferably reads the window descriptors from external memoryvia a direct memory access (DMA) module.

The DMA module may be shared by both paths of the display pipeline aswell as some of the control logic, such as the window controller and theCLUT. In order to support the display pipeline, the DMA modulepreferably has three channels where the graphics pipeline and the videopipeline use separate DMA modules. These may include window descriptorread, graphics data read and CLUT read. Each channel has externallyaccessible registers to control the start address and the number ofwords to read.

Once the DMA module has completed a transfer as indicated by its startand length registers, it preferably activates a signal that indicatesthe transfer is complete. This allows the DMA module that sets upoperations for that channel to begin setting up of another transfer. Inthe case of graphics data reads, the window controller preferably setsup a transfer of one line of graphics pixels and then waits for the DMAcontroller to indicate that the transfer of that line is complete beforesetting up the transfer of the next line, or of a line of anotherwindow.

Referring to FIG. 6, each window descriptor preferably includes four32-bit words (labeled Word 0 through Word 3) containing graphics windowdisplay information. Word 0 preferably includes a window operationparameter, a window format parameter and a window memory start address.The window operation parameter preferably is a 2-bit field thatindicates which operation is to be performed with the window descriptor.When the window operation parameter is 00b, the window descriptorperforms a normal display operation and when it is 01b, the windowdescriptor performs graphics color look-up table (“CLUT”) re-loading.The window operation parameter of 10b is preferably not used. The windowoperation parameter of 11b preferably indicates that the windowdescriptor is the last of a sequence of window descriptors in memory.

The window format parameter preferably is a 4-bit field that indicates adata format of the graphics data to be displayed in the graphics window.The data formats corresponding to the window format parameter isdescribed in Table 1 below.

TABLE 1 Graphics Data Formats win_(—) Data format Format Data FormatDescription 0000b RGB16 5-BIT RED, 6-BIT GREEN, 5-BIT BLUE 0001b RGB15 +1 RGB15 plus one bit alpha (keying) 0010b RGBA4444 4-BIT RED, GREEN,BLUE, ALPHA 0100b CLUT2 2-bit CLUT with YUV and alpha in table 0101bCLUT4 4-bit CLUT with YUV and alpha in table 0110b CLUT8 8-bit CLUT withYUV and alpha in table 0111b ACLUT16 8-BIT ALPHA, 8-BIT CLUT INDEX 1000bALPHA0 Single win_alpha and single RGB win_color 1001b ALPHA2 2-bitalpha with single RGB win_color 1010b ALPHA4 4-bit alpha with single RGBwin_color 1011b ALPHA8 8-bit alpha with single RGB win_color 1100bYUV422 U and V are sampled at half the rate of Y 1111b RESERVED Specialcoding for blank line in new header, i.e., indicates an empty line

The window memory start address preferably is a 26-bit data field thatindicates a starting memory address of the graphics data of the graphicswindow to be displayed on the screen. The window memory start addresspoints to the first address in the corresponding external SDRAM which isaccessed to display data on the graphics window defined by the windowdescriptor. When the window operation parameter indicates the graphicsCLUT reloading operation, the window memory start address indicates astarting memory address of data to be loaded into the graphics CLUT.

Word 1 in the window descriptor preferably includes a window layerparameter, a window memory pitch value and a window color value. Thewindow layer parameter is preferably a 4-bit data indicating the orderof layers of graphics windows. Some of the graphics windows may bepartially or completely stacked on top of each other, and the windowlayer parameter indicates the stacking order. The window layer parameterpreferably indicates where in the stack the graphics window defined bythe window descriptor should be placed.

In the preferred embodiment, a graphics window with a window layerparameter of 0000b is defined as the bottom most layer, and a graphicswindow with a window layer parameter of 1111b is defined as the top mostlayer. Preferably, up to eight graphics windows may be processed in eachscan line. The window memory pitch value is preferably a 12-bit datafield indicating the pitch of window memory addressing. Pitch refers tothe difference in memory address between two pixels that are verticallyadjacent within a window.

The window color value preferably is a 16-bit RGB color, which isapplied as a single color to the entire graphics window when the windowformat parameter is 1000b, 1001b, 1010b, or 1011b. Every pixel in thewindow preferably has the color specified by the window color value,while the alpha value is determined per pixel and per window asspecified in the window descriptor and the pixel format. The enginepreferably uses the window color value to implement a solid surface.

Word 2 in the window descriptor preferably includes an alpha type, awidow alpha value, a window y-end value and a window y-start value. Theword 2 preferably also includes two bits reserved for future definition,such as high definition television (HD) applications. The alpha type ispreferably a 2-bit data field that indicates the method of selecting analpha value for the graphics window. The alpha type of 00b indicatesthat the alpha value is to be selected from chroma keying. Chroma keyingdetermines whether each pixel is opaque or transparent based on thecolor of the pixel. Opaque pixels are preferably considered to have analpha value of 1.0, and transparent pixels have an alpha value of 0,both on a scale of 0 to 1. Chroma keying compares the color of eachpixel to a reference color or to a range of possible colors; if thepixel matches the reference color, or if its color falls within thespecified range of colors, then the pixel is determined to betransparent. Otherwise it is determined to be opaque.

The alpha type of 01b indicates that the alpha value should be derivedfrom the graphics CLUT, using the alpha value in each entry of the CLUT.The alpha type of 10b indicates that the alpha value is to be derivedfrom the luminance Y. The Y value that results from conversion of thepixel color to the YUV color space, if the pixel color is not already inthe YUV color, is used as the alpha value for the pixel. The alpha typeof 11b indicates that only a single alpha value is to be applied to theentire graphics window. The single alpha value is preferably included asthe window alpha value next.

The window alpha value preferably is an 8-bit alpha value applied to theentire graphics window. The effective alpha value for each pixel in thewindow is the product of the window alpha and the alpha value determinedfor each pixel. For example, if the window alpha value is 0.5 on a scaleof 0 to 1, coded as 0×80, then the effective alpha value of every pixelin the window is one-half of the value encoded in or for the pixelitself. If the window format parameter is 1000b, i.e., a single alphavalue is to be applied to the graphics window, then the per-pixel alphavalue is treated as if it is 1.0, and the effective alpha value is equalto the window alpha value.

The window y-end value preferably is a 10-bit data field that indicatesthe ending display line of the graphics window on the screen. Thegraphics window defined by the window descriptor ends at the displayline indicated by the window y-end value. The window y-start valuepreferably is a 10-bit data field that indicates a starting display lineof the graphics window on a screen. The graphics window defined by thewindow descriptor begins at the display line indicated in the windowy-start value. Thus, a display of a graphics window can start on anydisplay line on the screen based on the window y-start value.

Word 3 in the window descriptor preferably includes a window filterenable parameter, a blank start pixel value, a window x-size value and awindow x-start value. In addition, the word 3 includes two bits reservedfor future definition, such as HD applications. Five bits of the 32-bitword 3 are not used. The window filter enable parameter is a 1-bit fieldthat indicates whether low pass filtering is to be enabled during YUV4:4:4 to YUV 4:2:2 conversion.

The blank start pixel value preferably is a 4-bit parameter indicating anumber of blank pixels at the beginning of each display line. The blankstart pixel value preferably signifies the number of pixels of the firstword read from memory, at the beginning of the corresponding graphicswindow, to be discarded. This field indicates the number of pixels inthe first word of data read from memory that are not displayed. Forexample, if memory words are 32 bits wide and the pixels are 4 bitseach, there are 8 possible first pixels in the first word. Using thisfield, 0 to 7 pixels may be skipped, making the 1^(st) to the 8^(th)pixel in the word appear as the first pixel, respectively. The blankstart pixel value allows graphics windows to have any horizontalstarting position on the screen, and may be used during soft horizontalscrolling of a graphics window.

The window x-size value preferably is a 10-bit data field that indicatesthe size of a graphics window in the x direction, i.e., horizontaldirection. The window x-size value preferably indicates the number ofpixels of a graphics window in a display line.

The window x-start value preferably is a 10-bit data field thatindicates a starting pixel of the graphics window on a display line. Thegraphics window defined by the window descriptor preferably begins atthe pixel indicated by the window x-start value of each display line.With the window x-start value, any pixel of a given display line can bechosen to start painting the graphics window. Therefore, there is noneed to load pixels on the screen prior to the beginning of the graphicswindow display area with-black.

III. Graphics Window Control Data Passing Mechanism

In one embodiment of the present invention, a FIFO in the graphicsdisplay path accepts raw graphics data as the raw graphics data is readfrom memory, at the full memory data rate using a clock of the memorycontroller. In this embodiment, the FIFO provides this data, initiallystored in an external memory, to subsequent blocks in the graphicspipeline.

In systems such as graphics display systems where multiple types of datamay be output from one module, such as a memory controller subsystem,and used in another subsystem, such as a graphics processing subsystem,it typically becomes progressively more difficult to support acombination of dynamically varying data types and data transfer ratesand FIFO buffers between the producing and consuming modules. Theconventional way to address such problems is to design a logic block.that understands the varying parameters of the data types in the firstmodule and controls all of the relevant variables in the second module.This may be difficult due to variable delays between the two modules,due to the use of FIFOs between them and varying data rate, and due tothe complexity of supporting a large number of data types.

The system preferably processes graphics images for display byorganizing the graphics images into windows in which the graphics imagesappear on the screen, obtaining data that describes the windows, sortingthe data according to the depth of the window on the display,transferring graphics images from memory, and blending the graphicsimages using alpha values associated with the graphics images.

In the preferred embodiment, a packet of control information called aheader packet is passed from the window controller to the displayengine. All of the required control information from the windowcontroller preferably is conveyed to the display engine such that all ofthe relevant variables from the window controller are properlycontrolled in a timely fashion and such that the control is notdependent on variations in delays or data rates between the windowcontroller and the display engine.

A header packet preferably indicates the start of graphics data for onegraphics window. The graphics data for that graphics window continuesuntil it is completed without requiring a transfer of another headerpacket. A new header packet is preferably placed in the FIFO whenanother window is to start. The header packets may be transferredaccording to the order of the corresponding window descriptors in thewindow descriptor lists.

In a display engine that operates according to lists of windowdescriptors, windows may be specified to overlap one another. At thesame time, windows may start and end on any line, and there may be manywindows visible on any one line. There are a large number of possiblecombinations of window starting and ending locations along vertical andhorizontal axes and depth order locations. The system preferablyindicates the depth order of all windows in the window descriptorlistand implements the depth ordering correctly while accounting for allwindows.

Each window descriptor preferably includes a parameter indicating thedepth location of the associated window. The range that is allowed forthis parameter can be defined to be almost any useful value. In thepreferred embodiment there are 16 possible depth values, ranging from 0to 15, with 0 being the back-most (deepest, or furthest from theviewer), and 15 being the top or front-most depth. The windowdescriptors are ordered in the window descriptor list in order of thefirst display scan line where the window appears. For example if windowA spans lines 10 to 20, window B spans lines 12 to 18, and window Cspans lines 5 to 20, the order of these descriptors in the list would be{C, A, B}.

In the hardware, which is a preferably a VLSI device, there ispreferably on-chip memory capable of storing a number of windowdescriptors. In the preferred implementation, this memory can store upto 8 window descriptors on-chip, however the size of this memory may bemade larger or smaller without loss of generality. Window descriptorsare read from main memory into the on-chip descriptor memory in orderfrom the start of the list, and stopping when the on-chip memory is fullor when the most recently read descriptor describes a window that is notyet visible, i.e., its starting line is on a line that has a highernumber than the line currently being constructed. Once a window has beendisplayed and is no longer visible, it may be cast out of the on-chipmemory and the next descriptor in the list may read from main memory. Atany given display line, the order of the window descriptors in theon-chip memory bears no particular relation to the depth order of thewindows on the screen.

The hardware that controls the compositing of windows builds up thedisplay in layers, starting from the back-most layer. In the preferredembodiment, the back most layer is layer 0. The hardware performs aquick search of the back-most window descriptor that has not yet beencomposited, regardless of its location in the on-chip descriptor memory.In the preferred embodiment, this search is performed as follows:

All 8 window descriptors are stored on chip in such a way that the depthorder numbers of all of them are available simultaneously. While thedepth numbers in the window descriptors are 4 bit numbers, representing0 to 15, the on-chip memory has storage for 5 bits for the depth number.Initially the 5 bit for each descriptor is set to 0. The depth ordervalues are compared in a hierarchy of pair-wise comparisons, and thelower of the two depth numbers in each comparison wins the comparison.That is, at the first stage of the test descriptor pairs {0, 1}, {2, 3},{4, 5}, and {6, 7} are compared, where {0-7} represent the eightdescriptors stored in the on-chip memory. This results in four depthnumbers with associated descriptor numbers. At the next stage twopair-wise comparisons compare {(0, 1), (2, 3)} and {(4, 5), (6, 7)}.

Each of these results in a depth number of the lower depth order numberand the associated descriptor number. At the third stage, one pair-wisecomparison finds the smallest depth number of all, and its associateddescriptor number. This number points the descriptor in the on-chipmemory with the lowest depth number, and therefore the greatest depth,and this descriptor is used first to render the associated window on thescreen. Once this window has been rendered onto the screen for thecurrent scan line, the fifth bit of the depth number in the on-chipmemory is set to 1, thereby ensuring that the depth value number isgreater than 15, and as a result this depth number will preferably neveragain be found to be the back-most window until all windows have beenrendered on this scan line, preventing rendering this window twice.

Once all the windows have been rendered for a given scan line, the fifthbits of all the on-chip depth numbers are again set to 0; descriptorsthat describe windows that are no longer visible on the screen are castout of the on-chip memory; new descriptors are read from memory asrequired (that is, if all windows in the on-chip memory are visible, thenext descriptor is read from memory, and this repeats until the mostrecently read descriptor is not yet visible on the screen), and theprocess of finding the back most descriptor and rendering windows ontothe screen repeats.

Referring to FIG. 7, window descriptors are preferably sorted by thewindow controller and used to transfer graphics data to the displayengine. Each of window descriptors, including the window descriptor 0through the window descriptor 7 300 a-h, preferably contains a windowlayer parameter. In addition, each window descriptor is preferablyassociated with a window line done flag indicating that the windowdescriptor has been processed on a current display line.

The window controller preferably performs window sorting at each displayline using the window layer parameters and the window line done flags.The window controller preferably places the graphics window thatcorresponds to the window descriptor with the smallest window layerparameter at the bottom, while placing the graphics window thatcorresponds to the window descriptor with the largest window layerparameter at the top.

The window controller preferably transfers the graphics data for thebottom-most graphics window to be processed first. The window parametersof the bottom-most window are composed into a header packet and writtento the graphics FIFO. The DMA engine preferably sends a request to thememory controller to read the corresponding graphics data for thiswindow and send the graphics data to the graphics FIFO. The graphicsFIFO is then read by the display engine to compose a display line, whichis then written to graphics line buffers.

The window line done flag is preferably set true whenever the windowsurface has been processed on the current display line. The window linedone flag and the window layer parameter may be concatenated togetherfor sorting. The window line done flag is added to the window layerparameter as the most significant bit during sorting such that {windowline done flag[4 ], window layer parameter[3:0 ]} is a five bit binarynumber, a window layer value, with window line done flag as the mostsignificant bit.

The window controller preferably selects a window descriptor with thesmallest window layer value to be processed. Since the window line doneflag is preferably the most significant bit of the window layer value,any window descriptor with this flag set, i.e., any window that has beenprocessed on the current display line, will have a higher window layervalue than any of the other window descriptors that have not yet beenprocessed on the current display line. When a particular windowdescriptor is processed, the window line done flag associated with thatparticular window descriptor is preferably set high, signifying that theparticular window descriptor has been processed for the current displayline.

A sorter 304 preferably sorts all eight window descriptors after anywindow descriptor is processed. The sorting may be implemented usingbinary tree sorting or any other suitable sorting algorithm. In binarytree sorting for eight window descriptors, the window layer value forfour pairs of window descriptors are compared at a first level usingfour comparators to choose the window descriptor that corresponds to alower window in each pair. In the second level, two comparators are usedto select the window descriptor that corresponds to the bottom mostgraphics window in each of two pairs. In the third and the last level,the bottom-most graphics windows from each of the two pairs are comparedagainst each other preferably using only one comparator to select thebottom window.

A multiplexer 302 preferably multiplexes parameters from the windowdescriptors. The output of the sorter, i.e., window selected to be thebottom most, is used to select the window parameters to be sent to adirect memory access (“DMA”) module 306 to be packaged in a headerpacket and sent to a graphics FIFO 308. The display engine preferablyreads the header pack-et in the graphics FIFO and processes the rawgraphics data based on information contained in the header packet.

The header packet preferably includes a first header word and a secondheader word. Corresponding graphics data is preferably transferred asgraphics data words. Each of the first header word, the second headerword and the graphics data words preferably includes 32 bits ofinformation plus a data type bit. The first header word preferablyincludes a 1-bit data type, a 4-bit graphics type, a 1-bit first windowparameter, a 1-bit top/bottom parameter, a 2-bit alpha type, an 8-bitwindow alpha value and a 16-bit window color value. Table 2 showscontents of the first header word.

TABLE 2 First Header Word Bit 32 31-28 27 26 25- 23-16 15-0 Position 24Data Data graphics First top/ alpha window window Content type typeWindow bottom type alpha color

The 1-bit data type preferably indicates whether a 33-bit word in theFIFO is a header word or a graphics data word. A data type of 1indicates that the associated 33-bit word is a header word while thedata type of 0 indicates that the associated 33-bit word is a graphicsdata word. The graphics type indicates the data format of the graphicsdata to be displayed in the graphics window similar to the window formatparameter in the word 0 of the window descriptor, which is described inTable 1 above. In the preferred embodiment, when the graphics type is1111, there is no window on the current display line, indicating thatthe current display line is empty.

The first window parameter of the first header word preferably indicateswhether the window associated with that first header word is a firstwindow on a new display line. The top/bottom parameter preferablyindicates whether the current display line indicated in the first headerword is at the top or the bottom edges of the window. The alpha typepreferably indicates a method of selecting an alpha value individuallyfor each pixel in the window similar to the alpha type in the word 2 ofthe window descriptor.

The window alpha value preferably is an alpha value to be applied to thewindow as a whole and is similar to the window alpha value in the word 2of the window descriptor. The window color value preferably is the colorof the window in 16-bit RGB format and is similar to the window colorvalue in the word 1 of the window descriptor.

The second header word preferably includes the 1-bit data type, a 4-bitblank pixel count, a 10-bit left edge value, a 1-bit filter enableparameter and a 10-bit window size value. Table 3 shows contents of thesecond header word in the preferred embodiment.

TABLE 3 Second Header Word Bit 32 31-28 25-16 10 9-0 Position Data dataBlank pixel Left edge filter window size Content type count enabler

Similar to the first header word, the second header word preferablystarts with the data type indicating whether the second header word is aheader word or a graphics data word. The blank pixel count preferablyindicates a number of blank pixels at a left edge of the window and issimilar to the blank start pixel value in the word 3 of the windowdescriptor. The left edge preferably indicates a starting location ofthe window on a scan line, and is similar to the window x-start value inthe word 3 of the window descriptor. The filter enable parameterpreferably enables a filter during a conversion of graphics data from aYUV 4:4:4 format to a YUV 4:2:2 format and is similar to the windowfilter enable parameter in word 3 of the window descriptor. Some YUV4:4:4 data may contain higher frequency content than others, which maybe filtered-by enabling a low pass-filter during a conversion to the YUV4:2:2 format. The window size value preferably indicates the actualhorizontal size of the window and is similar to the window x-size valuein word 3 of the window descriptor.

When the composition of the last window of the last display line iscompleted, an empty-line header is preferably placed into the FIFO sothat the display engine may release the display line for display.

Packetized data structures have been used primarily in the communicationworld where large amount of data needs to be transferred betweenhardware using a physical data link (e.g., wires). The idea is not knownto have been used in the graphics world where localized and small datacontrol structures need to be transferred between different designentities without requiring a large off-chip memory as a buffer. In oneembodiment of the present system, header packets are used, and ageneral-purpose FIFO is used for routing. Routing may be accomplished ina relatively simple manner in the preferred embodiment because the writeport of the FIFO is the only interface.

In the preferred embodiment, the graphics FIFO is a synchronous 32×33FIFO built with a static dual-port RAM with one read port and one writeport. The write port preferably is synchronous to a 81 MHz memory clockwhile the read port may be asynchronous (not synchronized) to the memoryclock. The read port is preferably synchronous to a graphics processingclock, which runs preferably at 81 MHz, but not necessarily synchronizedto the memory clock. Two graphics FIFO pointers are preferablygenerated, one for the read port and one for the write port. In thisembodiment, each graphics FIFO pointer is a 6-bit binary counter whichranges from 000000b to 111111b, i.e., from 0 to 63. The graphics FIFO isonly 32 words deep and requires only 5 bits to represent each 33-bitword in the graphics FIFO. An extra bit is preferably used todistinguish between FIFO full and FIFO empty states.

The graphics data words preferably include the 1-bit data type and32-bit graphics data bits. The data type is 0 for the graphics datawords. In order to adhere to a common design practice that generallylimits the size of a DMA burst into a FIFO to half the size of the FIFO,the number of graphics data words in one DMA burst preferably does notexceed 16.

In an alternate embodiment, a graphics display FIFO is not used. In thisembodiment, the graphics converter processes data from memory at therate that it is read from memory. The memory and conversion functionsare in a same clock domain. Other suitable FIFO designs may be used.

Referring to FIG. 8, a flow diagram illustrates a process for loadingand processing window descriptors. First the system is preferably resetin step 310. Then the system in step 312 preferably checks for avertical sync (“VSYNC”). When the VSYNC is received, the system in step314 preferably proceeds to load window descriptors into the windowcontroller from the external SDRAM or other suitable memory over the DMAchannel for window descriptors. The window controller may store up toeight window descriptors in one embodiment of the present invention.

The step in step 316 preferably sends a new line header indicating thestart of a new display line. The system in step 320 preferably sorts thewindow descriptors in accordance with the process described in referenceto FIG. 7. Although sorting is indicated as a step in this flow diagram,sorting actually may be a continuous process of selecting thebottom-most window, i.e., the window to be processed. The system in step322 preferably checks to determine if a starting display line of thewindow is greater than the line count of the current display line. Ifthe starting display line of the window is greater than the line count,i.e., if the current display line is above the starting display line ofthe bottom most window, the current display line is a blank line. Thus,the system in step 318 preferably increments the line count and sendsanother new line header in step 316. The process of sending a new lineheader and sorting window descriptor continues as long as the startingdisplay line of the bottom most (in layer order) window is below thecurrent display line.

The display engine and the associated graphics filter preferably operatein one of two modes, a field mode and a frame mode. In both modes, rawgraphics data associated with graphics windows is preferably stored inframe format, including lines from both interlaced fields in the case ofan interlaced display. In the field mode, the display engine preferablyskips every other display line during processing. In the field mode,therefore, the system in step 318 preferably increments the line countby two each time to skip every other line. In the frame mode, thedisplay engine processes every display line sequentially. In the framemode, therefore, the system in step 318 preferably increments the linecount by one each time.

When the system in step 322 determines that the starting display of thewindow is greater than the line count, the system in step 324 preferablydetermines from the header packet whether the window descriptor is fordisplaying a window or re-loading the CLUT. If the window headerindicates that the window descriptor is for re-loading CLUT, the systemin step 328 preferably sends the CLUT data to the CLUT and turns on theCLUT write strobe to load CLUT.

If the system in step 324 determines that the window descriptor is fordisplaying a window, the system in step 326 preferably sends a newwindow header to indicate that graphics data words for a new window onthe display line are going to be transferred into the graphics FIFO.Then, the system in step 330 preferably requests the DMA module to sendgraphics data to the graphics FIFO over the DMA channel for graphicsdata. In the event the FIFO does not have sufficient space to storegraphics data in a new data packet, the system preferably waits untilsuch space is made available.

When graphics data for a display line of a current window is transferredto the FIFO, the system in step 332 preferably determines whether thelast line of the current window has been transferred. If the last linehas been transferred, a window descriptor done flag associated with thecurrent window is preferably set. The window descriptor done flagindicates that the graphics data associated with the current windowdescriptor has been completely transferred. When the window descriptordone flag is set, i.e., when the current window descriptor is completelyprocessed, the system sets a window descriptor done flag in step 334.Then the system in step 336 preferably sets a new window descriptorupdate flag and increments a window descriptor update counter toindicate that a new window descriptor is to be copied from the externalmemory.

Regardless of whether the last line of the current window has beenprocessed, the system in step 338 preferably sets the window line doneflag for the current window descriptor to signify that processing ofthis window descriptor on the current display line has been completed.The system in step 340 preferably checks the window line done flagsassociated with all eight window descriptors to determine whether theyare all set, which would indicate that all the windows of the currentdisplay line have been processed. If not all window line done flags areset, the system preferably proceeds to step 320 to sort. the windowdescriptors and repeat processing of the new bottom-most windowdescriptor.

If all eight window line done flags are determined to be set in step340, all window descriptors on the current display line have beenprocessed. In this case, the system in step 342 preferably checkswhether an all window descriptor done flag has been set to determinewhether all window descriptors have been processed completely. The allwindow descriptor done flag is set when processing of all windowdescriptors in the current frame or field have been processedcompletely. If the all window descriptor done flag is set, the systempreferably returns to step 310 to reset and awaits another VSYNC in step312. If not all window descriptors have been processed, the system instep 344 preferably determines if the new window descriptor update flaghas been set. In the preferred embodiment, this flag would have been setin step 334 if the current window descriptor has been completelyprocessed.

When the new window descriptor update flag is set, the system in step352 preferably sets up the DMA to transfer a new window descriptor fromthe external memory. Then the system in step 350 preferably clears thenew window descriptor update flag. After the system clears the newwindow descriptor update flag or when the new window descriptor updateflag is not set in the first place, the system in step 348 preferablyincrements a line counter to indicate that the window descriptors for anext display line should be processed. The system in step 346 preferablyclears all eight window line done flags to indicate that none of thewindow descriptors have been processed for the next display line. Thenthe system in step 316 preferably initiates processing of the newdisplay line by sending a new line header to the FIFO.

In the preferred embodiment, the graphics converter in the displayengine converts raw graphics data having various different formats intoa common format for subsequent compositing with video and for display.The graphics converter preferably includes a state machine that changesstate based on the content of the window data packet. Referring to FIG.9, the state machine in the graphics converter preferably controlsunpacking and processing of the header packets. A first header wordprocessing state 354 is preferably entered wherein a first windowparameter of the first header word is checked (step 356) to determine ifthe window data packet is for a first graphics window of a new line. Ifthe header packet is not for a first window of a new line, after thefirst header word is processed, the state preferably changes to a secondheader word processing state 362.

If the header packet is for a first graphics window of a new line, thestate machine preferably enters a clock switch state 358. In the clockswitch state, the clock for a graphics line buffer which is going tostore the new line switches from a display clock to a memory clock,e.g., from a 13.5 MHz clock to a 81 MHz clock. From the clock switchstate, a graphics type in the first header word is preferably checked(step 360 ) to determine if the header packet represents an empty line.A graphics type of 1111b preferably refers to an empty line.

If the graphics type is 1111b, the state machine enters the first headerword processing state 354, in which the first header word of the nextheader packet is processed. If the graphics type is not 1111b, i.e. thedisplay line is not empty, the second header word is processed. Then thestate machine preferably enters a graphics content state 364 whereinwords from the FIFO are checked (step 366 ) one at a time to verify thatthey are data words. The state machine preferably remains in thegraphics content state as long as each word read is a data word. Whilein the graphics content state, if a word received is not a data word,i.e., it is a first or second header word, then the state machinepreferably enters a pipeline complete state 368 and then to the firstheader processing state 354 where reading and processing of the nextwindow data packet is commenced.

Referring to FIG. 10, the display engine 58 is preferably coupled tomemory over a memory interface 370 and a CLUT over a CLUT interface 372.The display engine preferably includes the graphics FIFO 132 whichreceives the header packets and the graphics data from the memorycontroller over the memory interface. The graphics FIFO preferablyprovides received raw graphics data to the graphics converter 134 whichconverts the raw graphics data into the common compositing format.During the conversion of graphics format, the RGB to YUV converter 136and data from the CLUT over the CLUT interface 372 are used to convertRGB formatted data and CLUT formatted data, respectively.

The graphics converter preferably processes all of the window layers ofeach scan line in half the time, or less, of an interlaced display line,due to the need to have lines from both fields available in the SRAM foruse by the graphics filter when frame mode filtering is enabled. Thegraphics converter operates at 81 MHz in one embodiment of the presentinvention, and the graphics converter is able to process up to eightwindows on each scan line and up to three full width windows.

For example, with a 13.5 MHz display clock, if the graphics converterprocesses 81 Mpixels per second, it can convert three windows, eachcovering the width of the display, in half of the active display time ofan interlaced scan line. In one embodiment of the present invention, thegraphics converter processes all the window layers of each scan line inhalf the time of an interlaced display line, due to the need to havelines from both fields available in the SRAM for use by the graphicsfilter. In practice, there may be some more time available since theactive display time leaves out the blanking time, while the graphicsconverter can operate continuously.

Graphics pixels are preferably read from the FIFO in raw graphicsformat, using one of the multiple formats allowed in the presentinvention and specified in the window descriptor. Each pixel may occupyas little as two bits or as much as 16 bits in the preferred embodiment.Each pixel is converted to a YUVa24 format (also referred to as aYUV4:4:2:2), such as two adjacent pixels sharing a UV pair and havingunique Y and alpha values, and each of the Y, U, V and alpha componentsoccupying eight bits. The conversion process is generally dependent onthe pixel format type and the alpha specification method, both of whichare indicated by the window descriptor for the currently active window.Preferably, the graphics converter uses the CLUT memory to convert CLUTformat pixels into RGB or YUV pixels.

Conversions of RGB pixels may require conversion to YUV, and therefore,the graphics converter preferably includes a color space converter. Thecolor space converter preferably is accurate for all coefficients. Ifthe converter is accurate to eight or nine bits it can be used toaccurately convert eight bit per component graphics, such as CLUTentries with this level of accuracy or RGB24 images.

The graphics converter preferably produces one converted pixel per clockcycle, even when there are multiple graphics pixels packed into one wordof data from the FIFO. Preferably the graphics processing clock, whichpreferably runs at 81 MHz, is used during the graphics conversion. Thegraphics converter preferably reads data from the FIFO whenever bothconditions are met, including that the converter is ready to receivemore data, and the FIFO has data ready. The graphics converterpreferably receives an input from a graphics blender, which is the nextblock in the pipeline, which indicates when the graphics blender isready to receive more converted graphics data. The graphics convertermay stall if the graphics blender is not ready, and as a result, thegraphics converter may not be ready to receive graphics data from theFIFO.

The graphics converter preferably converts the graphics data into aYUValpha (“YUVa”) format. This YUVa format includes YUV 4:2:2 valuesplus an 8-bit alpha value for every pixel, and as such it occupies 24bits per pixel; this format is alternately referred to as aYUV 4:4:2:2.The YUV444-to-YUV422 converter 138 converts graphics data with the aYUV4:4:4:4 format from the graphics converter into graphics data with theaYUV 4:4:2:2 format and provides the data to the graphics blender 140.The YUV444-to-YUV422 converter preferably has a capacity of performinglow pass filtering to filter out high frequency components when needed.The graphics converter also sends and receives clock synchronizationinformation to and from the graphics line buffers over a clock controlinterface 376.

When provided with the converted graphics data, the graphics blender 140preferably composites graphics windows into graphics line buffers over agraphics line buffer interface 374. The graphics windows are alphablended into blended graphics and preferably stored in graphics linebuffers.

IV. Color Look-up Table Loading Mechanism

A color look-up table (“CLUT”) is preferably used to supply color andalpha values to the raw graphics data formatted to address informationcontents of the CLUT. For a window surface based display, there may bemultiple graphics windows on the same display screen with differentgraphics formats. For graphics windows using a color look-up table(CLUT) format, it may be necessary to load specific color look-up tableentries from external memory to on-chip memory before the graphicswindow is displayed.

The system preferably includes a display engine that processes graphicsimages formatted in a plurality of formats including a color look uptable (CLUT) format. The system provides a data structure that describesthe graphics in a window, provides a data structure that provides anindicator to load a CLUT, sorts the data structures into a listaccording to the location of the window on the display, and loadsconversion data into a CLUT for converting the CLUT-formatted data intoa different data format according to the sequence of data structures onthe list.

In the preferred embodiment, each window on the display screen isdescribed with a window descriptor. The same window descriptor is usedto control CLUT loading as the window descriptor used to displaygraphics on screen. The window descriptor preferably defines the memorystarting address of the graphics contents, the x position on the displayscreen, the width of the window, the starting vertical display line andend vertical display line, window layer, etc. The same window structureparameters and corresponding fields may be used to define the CLUTloading. For example, the graphics contents memory starting address maydefine CLUT memory starting address; the width of graphics windowparameter may define the number of CLUT entries to be loaded; thestarting vertical display line and ending vertical display lineparameters may be used to define when to load the CLUT; and the. windowlayer parameter may be used to define the priority of CLUT loading ifseveral windows are displayed at the same time, i.e., on the samedisplay line.

In the preferred embodiment, only one CLUT is used. As such, thecontents of the CLUT are preferably updated to display graphics windowswith CLUT formatted data that is not supported by the current content ofthe CLUT. One of ordinary skill in the art would appreciate that it isstraightforward to use more than one CLUT and switch back and forthbetween them for different graphics windows.

In the preferred embodiment, the CLUT is closely associated with thegraphics converter. In one embodiment of the present invention, the CLUTconsists of one SRAM with 256 entries and 32 bits per entry. In otherembodiments, the number of entries and bits per entry may vary. Eachentry contains three color components; either RGB or YUV format, and analpha component. For every CLUT-format pixel converted, the pixel datamay be used as the address to the CLUT and the resulting value may beused by the converter to produce the YUVa (or alternatively RGBa) pixelvalue.

The CLUT may be re-loaded by retrieving new CLUT data via the directmemory access module when needed. It generally takes longer to re-loadthe CLUT than the time available in a horizontal blanking interval.Accordingly, in the preferred embodiment, a whole scan line time isallowed to re-load the CLUT. While the CLUT is being reloaded, graphicsimages in non-CLUT formats may be displayed. The CLUT reloading ispreferably initiated by a window descriptor that contains informationregarding CLUT reloading rather than a graphics window displayinformation.

Referring to FIG. 11, the graphics CLUT 146 preferably includes agraphics CLUT controller 400 and a static dual-port RAM (SRAM) 402. TheSRAM preferably has a size of 256×32 which corresponds to 256 entries inthe graphics CLUT. Each entry in the graphics CLUT preferably has 32bits composed of Y+U+V+alpha from the most significant bit to the leastsignificant bit. The size of each field, including Y, U, V, and alpha,is preferably eight bits.

The graphics CLUT preferably has a write port that is synchronized to a81 MHz memory clock and a read port that may be asynchronous to thememory clock. The read port is preferably synchronous to the graphicsprocessing clock, which runs preferably at 81 MHz, but not necessarilysynchronized to the memory clock. During a read operation, the staticdual-port RAM (“SRAM”) is preferably addressed by a read address whichis provided by graphics data in the CLUT images. During the readoperation, the graphics data is preferably output as read data 414 whena memory address in the CLUT containing that graphics data is addressedby a read address 412.

During write operations, the window controller preferably controls thewrite port with a CLUT memory request signal 404 and a CLUT memory writesignal 408. CLUT memory data 410 is also preferably provided to thegraphics CLUT via the direct memory access module from the externalmemory. The graphics CLUT controller preferably receives the CLUT memorydata and provides the received CLUT memory data to the SRAM for writing.

Referring to FIG. 12, an exemplary timing diagram shows differentsignals involved during a writing operation of-the CLUT. The CLUT memoryrequest signal 418 is asserted when the CLUT is to be re-loaded. Arising edge of the CLUT memory request signal 418 is used to reset awrite pointer associated with the write port. Then the CLUT memory writesignal 420 is asserted to indicate the beginning of a CLUT re-loadingoperation. The CLUT memory data 422 is provided synchronously to the 81MHz memory clock 416 to be written to the SRAM. The write pointerassociated with the write port is updated each time the CLUT is loadedwith CLUT memory data.

In the preferred embodiment, the process of reloading a CLUT isassociated with the process of processing window descriptors illustratedin FIG. 8 since CLUT re-loading is initiated by a window descriptor. Asshown in steps 324 and 328 of FIG. B, if the window descriptor isdetermined to be for reloading CLUT in step 324, the system in step 328sends the CLUT data to the CLUT. The window descriptor for the CLUTreloading may appear anywhere in the window descriptor list.Accordingly, the CLUT reloading may take place at any time whenever CLUTdata is to be updated.

Using the CLUT loading mechanism in one embodiment of the presentinvention, more than one window with different CLUT tables may bedisplayed on the same display line. In this embodiment, only the minimumrequired entries are preferably loaded into the CLUT, instead of loadingall the entries every time. The loading of only the minimum requiredentries may save memory bandwidth and enables more functionality. TheCLUT loading mechanism is preferably relatively flexible and easy tocontrol, making it suitable for various applications. The CLUT loadingmechanism of the present invention may also simplify hardware design, asthe same state machine for the window controller may be used for CLUTloading. The CLUT preferably also shares the same DMA logic andlayer/priority control logic as the window controller.

V. Graphics Line Buffer Control Scheme

In the preferred embodiment of the present invention, the systempreferably blends a plurality of graphics images using line buffers. Thesystem initializes a line buffer by loading the line buffer with datathat represents transparent black, obtains control of a line buffer fora compositing operation, composites graphics contents into the linebuffer by blending the graphics contents with the existing contents ofthe line buffer, and repeats the step of compositing graphics contentsinto the line buffer until all of the graphics surfaces for theparticular line have been composited.

The graphics line buffer temporarily stores composited graphics images(blended graphics). A graphics filter preferably uses blended graphicsin line buffers to perform vertical filtering and scaling operations togenerate output graphics images. In the preferred embodiment, thedisplay engine composites graphics images line by line using a clockrate that is faster than the pixel display rate, and graphics filtersrun at the pixel display rate. In other embodiments, multiple lines ofgraphics images may be composited in parallel. In still otherembodiments, the line buffers may not be needed. Where line buffers areused, the system may incorporate an innovative control scheme forproviding the line buffers containing blended graphics to the graphicsfilter and releasing the line buffers that are used up by the graphicsfilter.

The line buffers are preferably built with synchronous static dual-portrandom access memory (“SRAM”) and dynamically switch their clocksbetween a memory clock and a display clock. Each line buffer ispreferably loaded with graphics data using the memory clock and thecontents of the line buffer is preferably provided to the graphicsfilter synchronously to the display clock. In one embodiment of thepresent invention, the memory clock is an 81 MHz clock used by thegraphics converter to process graphics data while the display clock is a13.5 MHz clock used to display graphics and video signals on atelevision screen. Other embodiments may use other clock speeds.

Referring to FIG. 13, the graphics line buffer preferably includes agraphics line buffer controller 500 and line buffers 504. The graphicsline buffer controller 500 preferably receives memory clock buffercontrol signals 508 as well as display clock buffer control signals 510.The memory clock control signals and the display clock control signalsare used to synchronize the graphics line buffers to the memory clockand the display clock, respectively. The graphics line buffer controllerreceives a clock selection vector 514 from the display engine to controlwhich graphics line buffers are to operate in which clock domain. Thegraphics line buffer controller returns a clock enable vector to thedisplay engine to indicate clock synchronization settings in accordancewith the clock selection vector.

In the preferred embodiment, the line buffers 504 include seven linebuffers 506 a-g. The line buffers temporarily store lines of YUVa24graphics pixels that are used by a subsequent graphics filter. Thisallows for four line buffers to be used for filtering and scaling, twoare available for progressing by one or two lines at the end of everyline, and one for the current compositing operation. Each line buffermay store an entire display line. Therefore, in this embodiment, thetotal size of the line buffers is (720 pixels/displayline)*(3bytes/pixel)*(7 lines)=15,120 bytes.

Each of the ports to the SRAM including line buffers is 24 bits wide toaccommodate graphics data in YUVa24 format in this embodiment of thepresent invention. The SRAM has one read port and one write port. Oneread port and one write port are used for the graphics blenderinterface, which performs a read-modify-write typically once per clockcycle. In another embodiment of the present invention, an SRAM with onlyone port is used. In yet another embodiment, the data stored in the linebuffers may be YUVa32 (4:4:4:4), RGBa32, or other formats. Those skilledin the art would appreciate that it is straightforward to vary thenumber of graphics line buffers, e.g., to use different number of tapsfor filter, the format of graphics data or the number of read and writeports for the SRAM.

The line buffers are preferably controlled by the graphics line buffercontroller over a line buffer control interface 502. Over thisinterface, the graphics line buffer controller transfers graphics datato be loaded to the line buffers. The graphics filter reads contents ofthe line buffers over a graphics line buffer interface 516 and clearsthe line buffers by loading them with transparent black pixels prior toreleasing them to be loaded with more graphics data for display.

Referring FIG. 14, a flow diagram of a process of using line buffers toprovide composited graphics data from a display engine to a graphicsfilter is illustrated. After the graphics display system is reset instep 520, the system in step 522 receives a vertical sync (VSYNC)indicating a field start. Initially, all line buffers preferably operatein the memory clock domain. Accordingly, the line buffers aresynchronized to the 81 MHz memory clock in one embodiment of the presentinvention. In other embodiments, the speed of the memory clock may bedifferent from 81 MHz, or the line buffers may not operate in the clockdomain of the main memory. The system in step 524 preferably resets allline buffers by loading them with transparent black pixels.

The system in step 526 preferably stores composited graphics data in theline buffers. Since all buffers are cleared at every field start by thedisplay engine to the equivalent of transparent black pixels, thegraphics data may be blended the same way for any graphics window,including the first graphics window to be blended. Regardless of howmany windows are composited into a line buffer, including zero windows,the result is preferably always the correct pixel data.

The system in step 528 preferably detects a horizontal sync (HSYNC)which signifies a new display line. At the start of each display line,the graphics blender preferably receives a line buffer release signalfrom the graphics filter when one or more line buffers are no longerneeded by the graphics filter. Since four line buffers are used with thefour-tap graphics filter at any given time, one to three line buffersare preferably made available for use by the graphics blender to beginconstructing new display lines in them. Once a line buffer releasesignal is recognized, an internal buffer usage register is updated andthen clock switching is performed to enable the display engine to workon the newly released one to three line buffers. In other embodiments,the. number of line buffers may be more or less than seven, and more orless than three line buffers may be released at a time.

The system in step 534 preferably performs clock switching. Clockswitching is preferably done in the memory clock domain by the displayengine using a clock selection vector. Each bit of the clock selectionvector preferably corresponds to one of the graphics line buffers.Therefore, in one embodiment of the present invention with sevengraphics line buffers, there are seven bits in the clock selectionvector. For example, a corresponding bit of logic 1 in the clockselection vector indicates that the line buffer operates in the memoryclock domain while a corresponding bit of logic 0 indicates that theline buffer operates in the display clock domain.

Other embodiments may have different numbers of line buffers and thenumber of bits in the clock selection vector may vary accordingly. Clockswitching logic preferably switches between the memory clock and thedisplay clock in accordance with the clock selection vector. The clockselection vector is preferably also used to multiplex the memory clockbuffer control signals and the display clock buffer control signals.

Since there is preferably no active graphics data at field and linestarts, clock switching preferably is done at the field start and theline start to accommodate the graphics filter to access graphics data inreal-time. At the field and line starts, clock switching may be donewithout causing glitches on the display side. Clock switching typicallyrequires a dead cycle time. A clock enable vector indicates that thegraphics line buffers are ready to synchronize to the clocks again. Theclock enable vector is preferably the same size at the clock selectionvector. The clock enable vector is returned to the display engine to becompared with the clock selection vector.

During clock switching, the clock selection vector is sent by thedisplay engine to the graphics line buffer block. The clocks arepreferably disabled to ensure a glitch-free clock switching. Thegraphics line buffers send the clock enable vector to the display enginewith the clock synchronization settings requested in the clock selectionvector. The display engine compares contents of the clock selectionvector and the clock enable vector. When the contents match, the clocksynchronization is preferably turned on again.

After the completion of clock switching during the video inactiveregion, the system in step 536 preferably provides the graphics data inthe line buffers to the graphics filter for anti-flutter filtering,sample rate conversion (SRC) and display. At the end of the currentdisplay line, the system looks for a VSYNC in step 538. If the VSYNC isdetected, the current field has been completed, and therefore, thesystem in step 530 preferably switches clocks for all line buffers tothe memory clock and resets the line buffers in step 524 for display ofanother field. If the VSYNC is not detected in step 538, the currentdisplay line is not the last display line of the current field. Thesystem continues to step 528 to detect another HSYNC for processing anddisplaying of the next display line of the current field.

VI. Window Soft Horizontal Scrolling Mechanism

Sometimes it is desirable to scroll a graphics window softly, e.g.,display text that moves from left to right or from right to leftsmoothly on a television screen. There are some difficulties that may beencountered in conventional methods that seek to implement horizontalsoft scrolling.

Graphics memory buffers are conventionally implemented using low-costDRAM, SDRAM, for example. Such memory devices are typically slow and mayrequire each burst transfer to be within a page. Smooth (or soft)horizontal scrolling, however, preferably enables the starting addressto be set to any arbitrary pixel. This may conflict with the transfer ofdata in bursts within the well-defined pages of DRAM. In addition,complex control logic may be required to monitor if page boundaries areto be crossed during the transfer of pixel maps for each step duringsoft horizontal scrolling.

In the preferred embodiment, an implementation of a soft horizontalscrolling mechanism is achieved by incrementally modifying the contentof a window descriptor for a particular graphics window. The window softhorizontal scrolling mechanism preferably enables positioning thecontents of graphics windows on arbitrary positions on a display line.

In an embodiment of the present invention, the soft horizontal scrollingof graphics windows is implemented based on an architecture in whicheach graphics window is independently stored in a normal graphics buffermemory device (SDRAM, EDO-DRAM, DRAM) as a separate object. Windows arecomposed on top of each other in real time as required. To scroll awindow to the left or right, a special field is defined in the windowdescriptor that tells how many pixels are to be shifted to the left orright.

The system according to the present invention provides a method ofhorizontally scrolling a display window to the left, which includes thesteps of blanking out one or more pixels at a beginning of a portion ofgraphics data, the portion being aligned with a start address; anddisplaying the graphics data starting at the first non-blanked out pixelin the portion of the graphics data aligned with the start address.

The system according to the present invention also provides a method ofhorizontally scrolling a display window to the right which includes thesteps of moving a read pointer to a new start address that isimmediately prior to a current start address, blanking out one or morepixels at a beginning of a portion of graphics data, the portion beingaligned to the new start address, and displaying the graphics datastarting at the first non-blanked out pixel in the portion of thegraphics data aligned with the new start address.

In practice, each graphics window is preferably addressed using aninteger word address. For example, if the memory system uses 32 bitwords, then the address of the start of a window is defined to bealigned to a multiple of 32 bits, even if the first pixel that isdesired to be displayed is not so aligned. Each graphics window alsopreferably has associated with it a horizontal offset parameter, inunits of pixels, that indicates a number of pixels to be ignored,starting at the indicated starting address, before the active display ofthe window starts. In the preferred embodiment, the horizontal offsetparameter is the blank start pixel value in the word 3 of the windowdescriptor. For example, if the memory system uses 32-bit words and thegraphics format of a window uses 8 bits per pixel, each 32-bit wordcontains four pixels. In this case, the display of the window may ignoreone, two or three pixels ( 8, 16, or 24 bits), causing an effective leftshift of one, two, or three pixels.

In the embodiment illustrated by the above example, the memory systemuses 32-bit words. In other embodiments, the memory system may use moreor less number of bits per word, such as 16 bits per word or 64 bits perword. In addition, pixels in other embodiments may have variousdifferent number of bits per pixel, such as 1, 2, 4, 8, 16, 24 and 32.

Referring to FIG. 15, in the preferred embodiment, a first pixel (e.g.,the first 8 bits) 604 of a 32-bit word 600, which is aligned to thestart address, is blanked out. The remaining three 8-bit pixels, otherthan the blanked out first pixel, are effectively shifted to the left byone pixel. Prior to blanking out, a read pointer 602 points to the firstbit of the 32-bit word. After blanking out, the read pointer 602 pointsto the ninth bit of the 32-bit word.

Further, a shift of four pixels is implemented by changing the startaddress by one to the next 32-bit word. Shifts of any number of pixelsare thereby implemented by a combination of adjusting the starting wordaddress and adjusting the pixel shift amount. The same mechanism may beused for any number of bits per pixel (1, 2, 4, etc.) and any memoryword size.

To shift a pixel or pixels to the right, the shifting cannot be achievedsimply, by blanking some of the bits at the start address since anyblanking at the start will simply have an effect of shifting pixels tothe left. Further, the shifting to the right cannot be achieved byblanking some of the bits at the end of the last data word of a displayline since display of a window starts at the start address regardless ofthe position of the last pixel to be displayed.

Therefore, in one embodiment of the present invention, when the graphicsdisplay is to be shifted to the right, a read pointer pointing at thestart address is preferably moved to an address that is just before thestart address, thereby making that address the new start address. Then,a portion of the data word aligned with the new start address is blankedout. This provides the effect of shifting the graphics display to theright.

For example, a memory system may use 32-bit words and the graphicsformat of a window may use 2 bits per pixel, e.g., a CLUT 2 format. Ifthe graphics display is to be shifted by a pixel to the right, the readpointer is moved to an address that is just before the start address,and that address becomes a new start address. Then, the first 30 bits ofthe 32-bit word that is aligned with the new start address are blankedout. In this case, blanking out of a portion of the 32-bit word that isaligned with the new start address has the effect of shifting thegraphics display to the right.

Referring to FIG. 16, a 32-bit word 610 that is aligned with thestarting address is shifted to the right by one pixel. The 32-bit word610 has a CLUT 2 format, and therefore contains 16 pixels. A readpointer 612 points at the beginning of the 32-bit word 610. To shift thepixels in the 32-bit word 610 to the right, an address that is justbefore the start address is made a new start address. A 32-bit data word618 is aligned with the new start address. Then, the first 30 bits (15pixels) 616 of the 32-bit data word 618 aligned with the new startaddress are blanked out. The read pointer 612 points at a new location,which is the 31^(st) bit of the new start address. The 31^(st) bit andthe 32^(nd) bit of the new start address may constitute a pixel 618.Insertion of the pixel 618 in front of 16 pixels of the 32-bit data word610 effectively shifts those 16 pixels to the right by one pixel.

VII. Anti-aliased Text and Graphics

TV-based applications, such as interactive program guides, enhanced TV,TV navigators, and web browsing on TV frequently require the display oftext and line-oriented graphics on the display. A graphical element orglyph generally represents an image of text or graphics. Graphicalelement may refer to text glyphs or graphics. In conventional methods ofdisplaying text on TV or computer displays, graphical elements arerendered as arrays of pixels (picture elements) with two states forevery pixel, i.e. the foreground and background colors.

In some cases the background color is transparent, allowing video orother graphics to show through. Due to the relatively low resolution ofmost present day TVs, diagonal and round edges of graphical elementsgenerally show a stair-stepped appearance which may be undesirable; andfine details are constrained to appear as one or more complete pixels(dots), which may not correspond well to the desired appearance. Theinterlaced nature of TV displays causes horizontal edges of graphicalelements, or any portion of graphical elements with a significantvertical gradient, to show a “fluttering” appearance with conventionalmethods.

Some conventional methods blend the edges of graphical elements withbackground colors in a frame buffer, by first reading the color in theframe buffer at every pixel where the graphical element will be written,combining that value with the foreground color of the graphical element,and writing the result back to the frame buffer memory. This methodrequires there to be a frame buffer; it requires the frame buffer to usea color format that supports such blending operations, such as RGB24 orRGB16, and it does not generally support the combination of graphicalelements over full motion video, as such functionality may requirerepeating the read, combine and write back function of all pixels of allgraphical elements for every frame or field of the video in a timelymanner.

The system preferably displays a graphical element by filtering thegraphical element with a low pass filter to generate a multi-level valueper pixel at an intended final display resolution and uses themulti-level values as alpha blend values for the graphical element inthe subsequent compositing stage.

In one embodiment of the present invention, a method of displayinggraphical elements on televisions and other displays is used. A deepcolor frame buffer with, for example, 16, 24, or 32 bits per pixel, isnot required to implement this method since this method is effectivewith as few as two bits per pixel. Thus, this method may result in asignificant reduction in both the memory space and the memory bandwidthrequired to display text and graphics. The method preferably provideshigh quality when compared with conventional methods of anti-aliasedtext, and produces higher display quality than is available withconventional methods that do not support anti-aliased text.

Referring to FIG. 17, a flow diagram illustrates a process of providingvery high quality display of graphical elements in one embodiment of thepresent invention. First, the bi-level graphical elements are filteredby the system in step 652. The graphical elements are preferablyinitially rendered by the system in step 650 at a significantly higherresolution than the intended final display resolution, for example, fourtimes the final resolution in both horizontal and vertical axes. Thefilter may be any suitable low pass filter, such as a “box” filter. Theresult of the filtering operation is a multi-level value per pixel atthe intended display resolution.

The number of levels may be reduced to fit the number of bits used inthe succeeding steps. The system in step 654 determines whether thenumber of levels are to be reduced by reducing the number of bits used.If the system determines that the number of levels are to be reduced,the system in step 656 preferably reduces the number of bits. Forexample, the result of box-filtering 4×4 super-sampled graphicalelements normally results in 17 possible levels; these may be convertedthrough truncation or other means to 16 levels to match a 4 bitrepresentation, or eight levels to match a 3 bit representation, or fourlevels to match a 2 bit representation. The filter may provide arequired vertical axis low pass filter function to provide anti-flutterfilter effect for interlaced display.

In step 658, the system preferably uses the resulting multi-levelvalues, either with or without reduction in the number of bits, as alphablend values, which are preferably pixel alpha component values, for thegraphical elements in a subsequent compositing stage. The multi-levelgraphical element pixels are preferably written into a graphics displaybuffer where the values are used as alpha blend values when the displaybuffer is composited with other graphics and video images.

In an alternate embodiment, the display buffer is defined to have aconstant foreground color consistent with the desired foreground colorof the text or graphics, and the value of every pixel in the displaybuffer is defined to be the alpha blend value for that pixel. Forexample, an Alpha-4 format specifies four bits per pixel of alpha blendvalue in a graphics window, where the 4 bits define alpha blend valuesof 0/16, 1/16, 2/16, . . . , 13/16, 14/16, and 16/16. The value 15/16 isskipped in this example in order to obtain the endpoint values of 0 and16/16 (1) without requiring the use of an additional bit. In thisexample format, the display window has a constant foreground color whichis specified in the window descriptor.

In another alternate embodiment, the alpha blend value per pixel isspecified for every pixel in the graphical element by choosing a CLUTindex for every pixel, where the CLUT entry associated with every indexcontains the desired alpha blend value as part of the CLUT contents. Forexample, a graphical element with a constant foreground color and 4 bitsof alpha per pixel can be encoded in a CLUT 4 format such that everypixel of the display buffer is defined to be a 4 bit CLUT index, andeach of the associated 16 CLUT entries has the appropriate alpha blendvalue (0/16, 1/16, 2/16, . . . , 14/16, 16/16) as well as the (same)constant foreground color in the color portion of the CLUT entries.

In yet another alternate embodiment, the alpha per pixel values are usedto form the alpha portion of color+alpha pixels in the display buffer,such as alphaRGB (4,4,4,4) with 4 bits for each of alpha, Red, Green,and Blue, or alphaRGB32 with 8 bits for each component. This format doesnot require the use of a CLUT.

In still another alternate embodiment, the graphical element may or maynot have a constant foreground color. The various foreground colors areprocessed using a low-pass filter as described earlier, and the outlineof the entire graphical element (including all colors other than thebackground) is separately filtered also using a low pass filter asdescribed. The filtered foreground color is used as either the directcolor value in, e.g., an alphaRGB format (or other color space, such asalphaYUV) or as the color choice in a CLUT format, and the result offiltering the outline is used as the alpha per pixel value in either adirect color format such as alphaRGB or as the choice of alpha value perCLUT entry in a CLUT format.

The graphical elements are displayed on the TV screen by compositing thedisplay buffer containing the graphical elements with optionally othergraphics and video contents while blending the subject display bufferwith all layers behind it using the alpha per pixel values created inthe preceding steps. Additionally, the translucency or opacity of theentire graphical element may be varied by specifying the alpha value ofthe display buffer via such means as the window alpha value that may bespecified in a window descriptor.

VIII. Video Synchronization

When a composite video signal (analog video) is received into thesystem, it is preferably digitized and separated into YUV (luma andchroma) components for processing. Samples taken for YUV are preferablysynchronized to a display clock for compositing with graphics data atthe video compositor. Mixing or overlaying of graphics with decodedanalog video may require synchronizing the two image sources exactly.Undesirable artifacts such as jitter may be visible on the displayunless a synchronization mechanism is implemented to correctlysynchronize the samples from the analog video to the display clock. Inaddition, analog video often does not adhere strictly to the televisionstandards such as NTSC and PAL. For example, analog video whichoriginates in VCRs may have synchronization signals that are not alignedwith chroma reference signals and also may have inconsistent lineperiods. Thus, the synchronization mechanism preferably should correctlysynchronize samples from non-standard analog videos as well.

The system, therefore, preferably includes a video synchronizingmechanism that includes a first sample rate converter for converting asampling rate of a stream of video samples to a first converted rate, afilter for processing at least some of the video samples with the firstconverted rate, and a second sample rate converter for converting thefirst converted rate to a second converted rate.

Referring to FIG. 18, the video decoder 50 preferably samples andsynchronizes the analog video input. The video receiver preferablyreceives an analog video signal 706 into an analog-to-digital converter(ADC) 700 where the analog video is digitized. The digitized analogvideo 708 is preferably sub-sampled by a chroma-locked sample rateconverter (SRC) 708. A sampled video signal 710 is provided to anadaptive 2H comb filter/chroma demodulator/luma processor 702 to beseparated into YUV (luma and chroma) components. In the 2H combfilter/chroma demodulator/luma processor 702, the chroma components aredemodulated. In addition, the luma component is preferably processed bynoise reduction, coring and detail enhancement operations. The adaptive2H comb filter provides the sampled video 712, which has been separatedinto luma and chroma components and processed, to a line-locked SRC 704.The luma and chroma components of the sample video is preferablysub-sampled once again by the line-locked SRC and the sub-sampled video714 is provided to a time base corrector (TBC) 72. The time basecorrector preferably provides an output video. signal 716 that issynchronized to a display clock of the graphics display system. In oneembodiment of the present invention, the display clock runs at a nominal13.5 MHz.

The synchronization mechanism preferably includes the chroma-locked SRC70, the line-locked SRC 704 and the TBC 72. The chroma-locked SRCoutputs samples that are locked to chroma subcarrier and its referencebursts while the line-locked SRC outputs samples that are locked tohorizontal syncs. In the preferred embodiment, samples of analog videoare over-sampled by the ADC 700 and then down-sampled by thechroma-locked SRC to four times the chroma sub-carrier frequency (Fsc).The down-sampled samples are down-sampled once again by the line-lockedSRC to line-locked samples with an effective sample rate of nominally13.5 MHz. The time base corrector is used to align these samples to thedisplay clock,. which runs nominally at 13.5 MHz.

Analog composite video has a chroma signal frequency interleaved infrequency with the luma signal. In an NTSC standard video, this chromasignal is modulated on to the Fsc of approximately 3.579545 MHz, orexactly 227.5 times the horizontal line rate. The luma signal covers afrequency span of zero to approximately 4.2 MHz. One method forseparating the luma from the chroma is to sample the video at a ratethat is a multiple of the chroma sub-carrier frequency, and use a combfilter on the sampled data. This method generally imposes a limitationthat the sampling frequency is a multiple of the chroma sub-carrierfrequency (Fsc).

Using such a chroma-locked sampling frequency generally imposessignificant costs and complications on the implementation, as it mayrequire the creation of a sample clock of the correct frequency, whichitself may require a stable, low noise controllable oscillator (e.g. aVCXO) in a control loop that locks the VCXO to the chroma burstfrequency. Different sample frequencies are typically required fordifferent video standards with different chroma subcarrier frequencies.Sampling at four times the subcarrier frequency, i.e. 14.318 MHz forNTSC standard and 17.72 MHz for PAL standard, generally requires moreanti-alias filtering before-digitization than is required when samplingat higher frequencies such as 27 MHz. In addition, such a chroma-lockedclock frequency is often unrelated to the other frequencies in a largescale digital device, requiring multiple clock domains and asynchronousinternal interfaces.

In the preferred embodiment, however, the samples are not taken at afrequency that is a multiple of Fsc. Rather, in the preferredembodiment, an integrated circuit takes samples of the analog video at afrequency that is essentially arbitrary and that is greater than fourtimes the Fsc (4 Fsc=14.318 MHz). The sampling frequency preferably is27 MHz and preferably is not locked to the input video signal in phaseor frequency. The sampled video data then goes through the chroma-lockedSRC that down-samples the data to an effective sampling rate of 4 Fsc.This and all subsequent operations are preferably performed in digitalprocessing in a single integrated circuit.

The effective sample rate of 4 Fsc does not require a clock frequencythat is actually at 4 Fsc, rather the clock frequency can be almost anyhigher frequency, such as 27 MHz, and valid samples occur on some clockcycles while the overall rate of valid samples is equal to 4 Fsc. Thedown-sampling (decimation) rate of the SRC is preferably controlled by achroma phase and frequency tracking module. The chroma phase andfrequency tracking module looks at the output of the SRC during thecolor burst time interval and continuously adjusts the decimation ratein order to align the color burst phase and frequency. The chroma phaseand frequency tracking module is implemented as a logical equivalent ofa phase locked loop (PLL), where the chroma burst phase and frequencyare compared in a phase detector to the effective sample rate, which isintended to be 4 Fsc, and the phase and frequency error terms are usedto control the SRC decimation rate.

The decimation function is applied to the incoming sampled video, andtherefore the decimation function controls the chroma burst phase andfrequency that is applied to the phase detector. This system is a closedfeedback loop (control loop) that functions in much the same way as aconventional PLL, and its operating parameters are readily designed inthe same way as those of PLLs.

Referring to FIG. 19, the chroma-locked SRC 70 preferably includes asample rate converter (SRC) 730, a chroma tracker 732 and a low passfilter (LPF). The SRC 730 is preferably a polyphase filter havingtime-varying coefficients. The SRC is preferably implemented with 35phases and the conversion ratio of 35/66. The SRC 730 preferablyinterpolates by exactly 35 and decimates by (66+epsilon), i.e. thedecimation rate is preferably adjustable within a range determined bythe minimum and maximum values of epsilon, generally a small range.Epsilon is a first adjustment value, which is used to adjust thedecimation rate of a first sample rate converter, i.e., thechroma-locked sample rate converter.

Epsilon is preferably generated by the control loop comprising thechroma tracker 732 and the LPF 734, and it can be negative, positive orzero. When the output samples of the SRC 730 are exactly frequency andphase locked to the color sub-carrier then epsilon is zero. The chromatracker tracks phase and frequency of the chroma bursts and comparesthem against an expected pattern.

In one embodiment of the present invention, the conversion rate of thechroma-locked SRC is adjusted so that, in effect, the SRC samples thechroma burst at exactly four times per chroma sub-carrier cycle. The SRCtakes the samples at phases 0 degrees, 90 degrees, 180 degrees and 270degrees of the chroma sub-carrier cycle. This means that a sample istaken at every cycle of the color sub-carrier at a zero crossing, apositive peak, zero crossing and a negative peak, (0, +1, 0, −1). If thepattern obtained from the samples is different from (0, +1, 0, −1), thisdifference is detected and the conversion ratio needs to be adjustedinside the control loop.

When the output samples of the chroma-locked SRC are lower in frequencyor behind in phase, e.g., the pattern looks like (−1, 0, +1, 0), thenthe chroma tracker 732 will make epsilon negative. When epsilon isnegative, the sample rate conversion ratio is higher than the nominal35/66, and this has the effect of increasing the frequency or advancingthe phase of samples at the output of the chroma-locked SRC. When theoutput samples of the chroma-locked SRC are higher in frequency orleading in phase, e.g., the pattern looks like (+1, 0, −1, 0), then thechroma tracker 732 will make epsilon positive. When epsilon is positive,the sample rate conversion ratio is lower than the nominal 35/66, andthis has the effect of decreasing the frequency or retarding the phaseof samples out of the chroma-locked SRC. The chroma tracker provideserror signal 736 to the LPF 734 that filters the error signal to filterout high frequency components and provides the filtered error signal tothe SRC to complete the control loop.

The sampling clock may run at the system clock frequency or at the clockfrequency of the destination of the decoded digital video. If thesampling clock is. running at the system clock, the cost of theintegrated circuit may be lower than one that has a system clock and asub-carrier locked video decoder clock. A one clock integrated circuitmay also cause less noise or interference to the analog-to-digitalconverter on the IC. The system is preferably all digital, and does notrequire an external crystal or a voltage controlled oscillator.

Referring to FIG. 20, an alternate embodiment of the chroma-locked SRC70 preferably varies the sampling rate while the conversion rate is heldconstant. A voltage controlled oscillator (e.g., VCXO) 760 varies thesampling rate by providing a sampling frequency signal 718 to the ADC700. The conversion rate in this embodiment is fixed at 35/66 in the SRC750 which is the ratio between four times the chroma sub-carrierfrequency and 27 MHz.

In this embodiment, the chroma burst signal at the output of thechroma-locked SRC is compared with the expected chroma burst signal in achroma tracker 752. The error signals 756 from the comparison betweenthe converted chroma burst and the expected chroma burst are passedthrough a low pass filter 754 and then filtered error signals 758 areprovided to the VCXO 760 to control the oscillation frequency of theVCXO. The oscillation frequency of the VCXO changes in response to thevoltage level of the provided error signals. Use of input voltage tocontrol the oscillation frequency of a VCXO is well known in the art.The system as described here is a form of a phase locked loop (PLL), thedesign and use of which is well known in the art.

After the completion of chroma-luma separation and other processing tothe chroma and luma components, the samples with the effective samplerate of 4 Fsc (i.e. 4 times the chroma subcarrier frequency) arepreferably decimated to samples with a sample rate of nominally 13.5 MHzthrough the use of a second sample rate converter. Since this samplerate is less than the electrical clock frequency of the digitalintegrated circuit in the preferred embodiment, only some clock cyclescarry valid data. In this embodiment, the sample rate is preferablyconverted to 13.5 MHz, and is locked to the horizontal line rate throughthe use of horizontal sync signals. Thus,. the second sample rateconverter is a line-locked sample rate converter (SRC).

The line-locked sample rate converter converts the current line of videoto a constant (Pout) number of pixels. This constant number of pixelsPout is normally 858 for ITU-R BT.601 applications and 780 for NTSCsquare pixel applications. The current line of video may have a variablenumber of pixels (Pin). In order to do this conversion from achroma-locked sample rate, the following steps are performed. The numberof input samples Pin of the current line of video is accuratelymeasured. This line measurement is used to calculate the sample rateconversion ratio needed to convert the line to exactly Pout samples. Anadjustment value to the sample rate conversion ratio is passed to asample rate converter module in the line-locked SRC to implement thecalculated sample rate conversion ratio for the current line. The sampleconversion ratio is calculated only once for each line. Preferably, theline-locked SRC also scales YUV components to the proper amplitudesrequired by ITU-R BT.601.

The number of samples detected in a horizontal line may be more or lessif the input video is a non-standard video. For example, if the incomingvideo is from a VCR, and the sampling rate is four times the colorsub-carrier frequency (4 Fsc), then the number of samples taken betweentwo horizontal syncs may be more or less than 910, where 910 is thenumber of samples per line that is obtained when sampling NTSC standardvideo at a sampling frequency of 4 Fsc. For example, the horizontal linetime from a VCR may vary if the video tape has been stretched.

The horizontal line time may be accurately measured by detecting twosuccessive horizontal syncs. Each horizontal sync is preferably detectedat the leading edge of the horizontal sync. In other embodiments, thehorizontal syncs may be detected by other means. For example, the shapeof the entire horizontal sync may be looked at for detection. In thepreferred embodiment, the sample rate for each line of video has beenconverted to four times the color sub-carrier frequency (4 Fsc) by thechroma-locked sample rate converter. The measurement of the horizontalline time is preferably done at two levels of accuracy, an integer pixelaccuracy and a sub-sample accuracy.

The integer pixel accuracy is preferably done by counting the integernumber of pixels that occur between two successive sync edges. The syncedge is presumed to be detected when the data crosses some thresholdvalue. For example, in one embodiment of the present invention, theanalog-to-digital converter (ADC) is a 10-bit ADC, i.e., converts aninput analog signal into a digital signal with (2^10−1=1023) scalelevels. In this embodiment, the threshold value is chosen to representan appropriate slicing level for horizontal sync in the 10-bit numbersystem of the ADC; a typical value for this threshold is 128. Thenegative peak (or a sync tip) of the digitized video signal normallyoccurs during the sync pulses. The threshold level would normally be setsuch that it occurs at approximately the mid-point of the sync pulses.The threshold level may be automatically adapted by the video decoder,or it may be set explicitly via a register or other means.

The horizontal sync tracker preferably detects the horizontal sync edgeto a sub-sample accuracy of ( 1/16)th of a pixel in order to moreaccurately calculate the sample rate conversion. The incoming samplesgenerally do not include a sample taken exactly at the threshold valuefor detecting horizontal sync edges. The horizontal sync trackerpreferably detects two successive samples, one of which has a valuelower than the threshold value and the other of which has a value higherthan the threshold value.

After the integer pixel accuracy is determined (sync edge has beendetected) the sub-pixel calculation is preferably started. The sync edgeof a horizontal sync is generally not a vertical line, but has a slope.In order to remove noise, the video signal goes through a low passfilter. The low pass filter generally decreases sharpness of thetransition, i.e., the low pass filter may make the transition from a lowlevel to a high level last longer.

The horizontal sync tracker preferably uses a sub-sample interpolationtechnique to obtain an accurate measurement of sync edge location bydrawing a straight line between the two successive samples of thehorizontal sync signal just above and just below the presumed thresholdvalue to determine where the threshold value has been crossed.

Three values are preferably used to determine the sub-sample accuracy.The three values are the threshold level (T), the value of the samplethat crossed the threshold level (V2) and the value of the previoussample that did not cross the threshold level (V1). The sub-sample valueis the ratio of (T−V1)/(V2−V1). In the present embodiment a division isnot performed. The difference (V2−V1) is divided by 16 to make avariable called DELTA. V1 is then incremented by DELTA until it exceedsthe threshold T. The number of times that DELTA is added to V1 in orderto make it exceed the threshold (T) is the sub-pixel accuracy in termsof 1/16^(th) of a pixel.

For example, if the threshold value T is presumed to be 146 scalelevels, and if the values V1 and V2 of the two successive samples are140 and 156, respectively, the DELTA is calculated to be 1, and thecrossing of the threshold value is determined through interpolation tobe six DELTAs away from the first of the two successive samples. Thus,if the sample with value 140 is the nth sample and the sample with thevalue 156 is the (n+1)th sample, the (n+( 6/16))th sample would have hadthe threshold value. Since the horizontal sync preferably is presumed tobe detected at the threshold value of the sync edge, a fractionalsample, i.e., 6/16 sample, is added to the number of samples countedbetween two successive horizontal syncs.

In order to sample rate convert the current number of input pixels Pinto the desired output pixels Pout, the sample rate converter module hasa sample rate conversion ratio of Pin/Pout. The sample rate convertermodule in the preferred embodiment of the line-locked sample rateconverter is a polyphase filter with time-varying coefficients. There isa fixed number of phases (I) in the polyphase filter. In the preferredembodiment, the number of phases (I) is 33. The control for thepolyphase filter is the decimation rate (d_act) and a reset phasesignal. The line measurement Pin is sent to a module that converts it toa decimation rate d_act such that I/d_act (33/d_act) is equal toPin/Pout. The decimation rate d_act is calculated as follows:d_act=(I/Pout)*Pin.

If the input video line is the standardized length of time and the fourtimes the color sub-carrier is the standardized frequency then Pin willbe exactly 910 samples. This gives a sample rate conversion ratio of(858/910). In the present embodiment the number of phases (theinterpolation rate) is 33. Therefore the nominal decimation rate forNTSC is 35 (=(33/858)*910). This decimation rate d_act may then be sentto the sample rate converter module. A reset phase signal is sent to thesample rate converter module after the sub-sample calculation has beendone and the sample rate converter module starts processing the currentvideo line. In the preferred embodiment, only the active portion ofvideo is processed and sent on to a time base corrector. This results ina savings of memory needed. Only 720 samples of active video areproduced as ITU-R BT.601 output sample rates. In other embodiments, theentire horizontal line may be processed and produced as output.

In the preferred embodiment, the calculation of the decimation rated_act is done somewhat differently from the equation d_act=(I/Pout)*Pin.The results are the same, but there are savings to hardware. The currentline length, Pin, will have a relatively small variance with respect tothe nominal line length. Pin is nominally 910. It typically varies byless than 62. For NTSC, this variation is less than 5 microseconds. Thefollowing calculation is done:d_act=((I/Pout)*(Pin−Pin_nominal))+d_act_nominal

This preferably results in a hardware savings for the same level ofaccuracy. The difference (Pin−Pin_nominal) may be represented by fewerbits than are required to represent Pin so a smaller multiplier can-beused. For NTSC, d_act_nominal is 35 and Pin_nominal is 910. The value(I/Pout)*(Pin−Pin_nominal) may now be called a delta_dec (deltadecimation rate) or a second adjustment value.

Therefore, in order to maintain the output sample rate of 858 samplesper horizontal line, the conversion rate applied preferably is33/(35+delta_dec) where the samples are interpolated by 33 and decimatedby (35+delta_dec). A horizontal sync tracker preferably detectshorizontal syncs, accurately counts the number of samples between twosuccessive horizontal syncs and generates delta_dec.

If the number of samples between two successive horizontal syncs isgreater than 910, the horizontal sync tracker generates a positivedelta_dec to keep the output sample rate at 858 samples per horizontalline. On the other hand, if the number of samples between two successivehorizontal syncs is less than 910, the horizontal sync tracker generatesa negative delta_dec to keep the output sample rate at 858 samples perhorizontal line.

For PAL standard video, the horizontal sync tracker generates thedelta_dec to keep the output sample rate at 864 samples per horizontalline.

In summary, the position of each horizontal sync pulse is determined tosub-pixel accuracy by interpolating between two successive samples, oneof which being immediately below the threshold value and the other beingimmediately above the threshold value. The number of samples between thetwo successive horizontal sync pulses is preferably calculated tosub-sample accuracy by determining the positions of two successivehorizontal sync pulses, both to sub-pixel accuracy. When calculatingdelta_dec, the horizontal sync tracker preferably uses the differencebetween 910 and the number of samples between two successive horizontalsyncs to reduce the amount of hardware needed.

In an alternate embodiment, the decimation rate adjustment value,delta_dec, which is calculated for each line, preferably goes through alow pass filter before going to the sample rate converter module. One ofthe benefits of this method is filtering of variations in the linelengths of adjacent lines where the variations may be caused by noisethat affects the accuracy of the measurement of the sync pulsepositions.

In another alternative embodiment, the input sample clock is not freerunning, but is instead line-locked to the input analog video,preferably 27 HMz. The chroma-locked sample rate converter converts the27 HMz sampled data to a sample rate of four times the color sub-carrierfrequency. The analog video signal is demodulated to luma and chromacomponent video signals, preferably using a comb filter. The luma andchroma component video signals are then sent to the line-locked samplerate converter where they are preferably converted to a sample rate of13.5 MHz. In this embodiment the 13.5 MHz sample rate at the output maybe exactly one-half of the 27 HMz sample rate at the input. Theconversion ratio of the line-locked sample rate converter is preferablyexactly one-half of the inverse of the conversion ratio performed by thechroma-locked sample rate converter.

Referring to FIG. 21, the line-locked SRC 704 preferably includes an SRC770 which preferably is a polyphase filter with time varyingcoefficients. The number of phases is preferably fixed at 33 while thenominal decimation rate is 35. In other words, the conversion ratio usedis preferably 33/(35+delta_dec) where delta_dec may be positive ornegative. The delta_dec is a second adjustment value, which is used toadjust the decimation rate of the second sample rate converter.Preferably, the actual decimation rate and phase are automaticallyadjusted for each horizontal line so that the number of samples perhorizontal line is 858 (720 active Y samples and 360 active U and Vsamples) and the phase of the active video samples is aligned properlywith the horizontal sync signals.

In the preferred embodiment, the decimation (down-sampling) rate of theSRC is preferably controlled by a horizontal sync tracker 772.Preferably, the horizontal sync tracker adjusts the decimation rate onceper horizontal line in order to result in a correct number and phase ofsamples in the interval between horizontal syncs. The horizontal synctracker preferably provides the adjusted decimation rate to the SRC 770to adjust the conversion ratio. The decimation rate is preferablycalculated to achieve a sub-sample accuracy of 1/16. Preferably, theline-locked SRC 704 also includes a YUV scaler 780 to scale YUVcomponents to the proper amplitudes required by ITU-R BT.601.

The time base corrector (TBC) preferably synchronizes the samples havingthe line-locked sample rate of nominally 13.5 MHz to the display clockthat runs nominally at 13.5 MHz. Since the samples at the output of theTBC are synchronized to the display clock, passthrough video may beprovided to the video compositor without being captured first.

To produce samples at the sample rate of nominally 13.5 MHz, thecomposite video may be sampled in any conventional way with a clock ratethat is generally used in the art. Preferably, the composite video issampled initially at 27 HMz, down sampled to the sample rate of 14.318MHz by the chroma-locked SRC, and then down sampled to the sample rateof nominally 13.5 MHz by the line-locked SRC. During conversion of thesample rates, the video decoder uses for timing the 27 HMz clock thatwas used for input sampling. The 27 HMz clock, being free-running, isnot locked to the line rate nor to the chroma frequency of the incomingvideo.

In the preferred embodiment, the decoded video samples are stored in aFIFO the size of one display line of active video at 13.5 MHz, i.e., 720samples with 16 bits per sample or 1440 bytes. Thus, the maximum delayamount of this FIFO is one display line time with a normal, nominaldelay of one-half a display line time. In the preferred embodiment,video samples are outputted from the FIFO at the display clock rate thatis nominally 13.5 MHz. Except for vertical syncs of the input video, thedisplay clock rate is unrelated to the timing of the input video. Inalternate embodiments, larger or smaller FIFOs may be used.

Even though the effective sample rate and the display clock rate areboth nominally 13.5 MHz the rate of the sampled video entering the FIFOand the display rate are generally different. This discrepancy is due todifferences between the actual frequencies of the effective input samplerate and the display clock. For example, the effective input sample rateis nominally 13.5 MHz but it is locked to operate at 858 times the linerate of the video input, while the display clock operates nominally at13.5 MHz independently of the line rate of the video input.

Since the rates of data entering and leaving the FIFO are typicallydifferent, the FIFO will tend to either fill up or become empty,depending on relative rates of the entering and leaving data. In oneembodiment of the present invention, video is displayed with an initialdelay of one-half a horizontal line time at the start of every field.This allows the input and output rates to differ up to the point wherethe input and output horizontal phases may change by up to one-half ahorizontal line time without causing any glitches at the display.

The FIFO is preferably filled up to approximately one-half full duringthe first active video line of every field prior to taking any outputvideo. Thus, the start of each display field follows the start of everyinput video field by a fixed delay that is approximately equal toone-half the amount of time for filling the entire FIFO. As such, theinitial delay at the start of every field is one-half a horizontal linetime in this embodiment, but the initial delay may be different in otherembodiments.

Referring to FIG. 22, the time base corrector (TBC) 72 includes a TBCcontroller 164 and a FIFO 166. The FIFO 166 receives an input video 714at nominally 13.5 MHz locked to the horizontal line rate of the inputvideo and outputs a delayed input video as an output video 716 that islocked to the display clock that runs nominally at 13.5 MHz. The initialdelay between the input video and the delayed input video is half ahorizontal line period of active video, e.g., 53.5 μs per active videoin a horizontal line/2=26.75 μs for NTSC standard video.

The TBC controller 164 preferably generates. a vertical sync (VSYNC) fordisplay that is delayed by one-half a horizontal line from an inputVSYNC. The TBC controller 164 preferably also generates timing signalssuch as NTSC or PAL standard timing signals. The timing signals arepreferably derived from the VSYNC generated by the TBC controller andpreferably include horizontal sync. The timing signals are not affectedby the input video, and the FIFO is read out synchronously to the timingsignals. Data is read out of the FIFO according to the timing at thedisplay side while the data is written into the FIFO according to theinput timing. A line reset resets the FIFO write pointer to signal a newline. A read pointer controlled by the display side is updated by thedisplay timing.

As long as the accumulated change in FIFO fullness, in either direction,is less than one-half a video line, the FIFO will generally neitherunderflow nor overflow during the video field. This ensures correctoperation when the display clock frequency is anywhere within a fairlybroad range centered on the nominal frequency. Since the process isrepeated every field, the FIFO fullness changes do not accumulate beyondone field time.

Referring to FIG. 23, a flow diagram of a process using the TBC 72 isillustrated. The process resets in step 782 at system start up. Thesystem preferably checks for vertical sync (VSYNC) of the input video instep 784. After receiving the input VSYNC, the system in step 786preferably starts counting the number of incoming video samples. Thesystem preferably loads the FIFO in step 788 continuously with theincoming video samples. While the FIFO is being loaded, the system instep 790 checks if enough samples have been received to fill the FIFO upto a half full state.

When enough samples have been received to fill the FIFO to the half fullstate, the system in step 792 preferably generates timing signalsincluding horizontal sync to synchronize the output of the TBC to thedisplay clock. The system in step 794 preferably outputs the content ofthe FIFO continuously in sync with the display clock. The system in step796 preferably checks for another input VSYNC. When another inputvertical sync is detected, the process starts counting the number ofinput video samples again and starts outputting output video sampleswhen enough input video samples have been received to make the FIFO halffull.

In other embodiments of the present invention, the FIFO size may besmaller or larger. The minimum size acceptable is determined by themaximum expected difference in the video source sample rate and thedisplay sample rate. Larger FIFOs allow for greater variations in samplerate timing, however at greater expense. For any chosen FIFO size, thelogic that generates the sync signal that initiates display video fieldsshould incur a delay from the input video timing of one-half the delayof the entire FIFO as described above. However, it is not required thatthe delay be one-half the delay of the entire FIFO.

IX. Video Scaler

In certain applications of graphics and video display hardware, it maybe necessary or desirable to scale the size of a motion video imageeither upwards or downwards. It may also be desirable to minimize memoryusage and memory bandwidth demands. Therefore it is desirable to scaledown before writing to memory, and to scale up after reading frommemory, rather than the other way around in either case. Conventionallythere is either be separate hardware to scale down before writing tomemory and to scale up after reading from memory, or else all scaling isdone in one location or the other, such as before writing to memory,even if the scaling direction is upwards.

In the preferred embodiment, a video scaler performs both scaling-up andscaling-down of either digital video or digitized analog video. Thevideo scaler is preferably configured such that it can be used foreither scaling down the size of video images prior to writing them tomemory or for scaling up the size of video images after reading themfrom memory. The size of the video images are preferably downscaledprior to being written to memory so that the memory usage and the memorybandwidth demands are minimized. For similar reasons, the size of thevideo images are preferably upscaled after reading them from memory.

In the former case, the video scaler is preferably in the signal pathbetween a video input and a write port of a memory controller. In thelatter case, the video scaler is preferably in the signal path between aread port of the memory controller and a video compositor. Therefore,the video scaler may be seen to exist in two distinct logical places inthe design, while in fact occupying only one physical implementation.

This function is preferably achieved by arranging a multiplexingfunction at the input of the scaling engine, with one input to themultiplexer being connected to the video input port and the otherconnected to the memory read port. The memory write port is arrangedwith a multiplexer at its input, with one input to the multiplexerconnected to the output of the. scaling engine and the other connectedto the video input port. The-display output port is arranged with amultiplexer at its input, with one connected to the output of thescaling engine and the other input connected to the output of the memoryread port.

In the preferred embodiment, there are different clock domainsassociated with the video input and the display output functions of thechip. The video scaling engine uses a clock that is selected between thevideo input clock and the display output clock (display clock). Theclock selection uses a glitch-free clock selection logic, i.e. a circuitthat prevents the creation of extremely narrow clock pulses when theclock selection is changed. The read and write interfaces to memory bothuse asynchronous interfaces using FIFOs,.so the memory clock domain maybe distinct from both the video input clock domain and the displayoutput clock domain.

Referring to FIG. 24, a flow diagram illustrates a process ofalternatively upscaling or downscaling the video input 800. The systemin. step 802 preferably selects between a downscaling operation and anupscaling operation. If the downscaling operation is selected, thesystem in step 804 preferably downscales the input video prior tocapturing the input video in memory in step 806. If the upscalingoperation is selected in step 802, the system in step 806 preferablycaptures the input video in memory without scaling it.

Then the system in step 808 outputs the downscaled video as downscaledoutput 810. The system in step 808, however, sends non-scaled video inthe upscale path to be upscaled in step 812. The system in step 812upscales the non-scaled video and outputs it as upscaled video output814.

The video pipeline preferably supports up to one scaled video window andone passthrough video window, plus one background color, all of whichare logically behind the set of graphics windows. The order of thesewindows, from back to front, is fixed as background, then passthrough,then scaled video. The video windows are preferably always in YUVformat, although they can be in either 4:2:2 or 4:2:0 variants of YUV.Alternatively they can be in RGB or other formats.

When digital video, e.g., MPEG is provided to the graphics displaysystem or when analog video is digitized, the digital video Or thedigitized analog video is provided to a video compositor using one ofthree signal paths, depending on processing requirements. The digitalvideo and the digitized analog video are provided to the videocompositor as passthrough video over a passthrough path, as upscaledvideo over an upscale path and a downscaled video over a downscale path.

Either of the digital video or the analog video may be provided to thevideo compositor as the passthrough video while the other of the digitalvideo or the analog video is provided as an upscaled video or adownscaled video. For example, the digital video may be provided to thevideo compositor over the passthrough path while, at the same time, thedigitized analog video is downscaled and provided to the videocompositor over the downscale path as a video window. In one embodimentof the present invention where the scaler engine is shared between theupscale path and the downscale path, the scaler engine may upscale videoin either the vertical or horizontal axis while downscaling video in theother axis. However, in this embodiment, an upscale operation and adownscale operation on the same axis are not performed at the same timesince only one filter is used to perform both upscaling and downscalingfor each axis.

Referring to FIG. 24 a single video scaler 52 preferably performs boththe downscaling and upscaling operations. In particular, signals of thedownscale path only are illustrated. The video scaler 52 includes ascaler engine 182, a set of line buffers 178, a vertical coefficientmemory 180 A and a horizontal coefficient memory 180 B. The scalerengine 182 is implemented as a set of two polyphase filters, one foreach of horizontal and vertical dimensions.

In one embodiment of the present invention, the vertical polyphasefilter is a four-tap filter with programmable coefficients from thevertical coefficient memory 180 A. In other embodiments, the number oftaps in the vertical polyphase filter may vary. In one embodiment of thepresent invention, the horizontal polyphase filter is an eight-tapfilter with programmable coefficients from the horizontal coefficientmemory 180B. In other embodiments, the number of taps in the horizontalpolyphase filter may vary.

The vertical and the horizontal coefficient memories may be implementedin SRAM or any other suitable memory. Depending on the operation to beperformed, e.g. a vertical or horizontal axis, and scaling-up orscaling-down, appropriate filter coefficients are used, respectively,from the vertical and horizontal coefficient memories. Selection offilter coefficients for scaling-up and scaling-down operations are wellknown in the art.

The set of line buffers 178 are used to provide input of video data tothe horizontal and vertical polyphase filters. In this embodiment, threeline buffers are used, but the number of the line buffers may vary inother embodiments. In this embodiment, each of the three line buffers isused to provide an input to one of the taps of the vertical polyphasefilter with four taps. The input video is provided to the fourth tap ofthe vertical polyphase filter. A shift register having eight cells inseries is used to provide inputs to the eight taps of the horizontalpolyphase filter, each cell providing an input to one of the eight taps.

In this embodiment, a digital video signal 820 and a digitized analogsignal video 822 are provided to a first multiplexer 168 as first andsecond inputs. The first multiplexer 168 has two outputs. A first outputof the first multiplexer is provided to the video compositor as a passthrough video 186. A second output of the first multiplexer is providedto a first input of a second multiplexer 176 in the downscale path.

In the downscale path, the second multiplexer 176 provides either thedigital video or the digitized analog video at the second multiplexer'sfirst input to the video scaler 52. The video scaler provides adownscaled video signal to a second input of a third multiplexer 162.The third multiplexer provides the downscaled video to a capture FIFO158 which stores the captured downscaled video. The memory controller126 takes the captured downscaled video and stores it as a captureddownscaled video image into a video FIFO 148. An output of the videoFIFO is coupled to a first input of a fourth multiplexer 188. The fourthmultiplexer provides the output of the video FIFO, which is the captureddownscaled video image, as an output 824 to the graphics compositor, andthis completes the downscale path. Thus, in the downscale path, eitherthe digital video or the digitized analog video is downscaled first, andthen captured.

FIG. 26 is similar to FIG. 25, but in FIG. 26, signals of the upscalepath are illustrated. In the upscale path, the third multiplexer 162provides either the digital video 820 or the digitized analog video 822to the capture FIFO 158 which captures and stores input as a capturedvideo image. This captured video image is provided to the memorycontroller 126 which takes it and provides to the video FIFO 148 whichstores the captured video image.

An output of the video FIFO 148 is provided to a second input of thesecond multiplexer 176. The second multiplexer provides the capturedvideo image to the video scaler 52. The video scaler scales up thecaptured video image and provides it to a second input of the fourthmultiplexer 188 as an upscaled captured video image. The fourthmultiplexer provides the upscaled captured video image as the output 824to the video compositor. Thus, in the upscale path, either the digitalvideo or the digitized analog video is captured first, and thenupscaled.

Referring to FIG. 27, FIG. 27 is similar to FIG. 25 and FIG. 26, but inFIG. 27, signals of both the upscale path and the downscale path areillustrated.

X. Blending of Graphics and Video Surfaces

The graphics display system of the present invention is capable ofprocessing an analog video signal, a digital video signal and graphicsdata simultaneously. In the graphics display system, the analog anddigital video signals are processed in the video display pipeline whilethe graphics data is processed in the graphics display pipeline. Afterthe processing of the video signals and the graphics data have beencompleted, they are blended together at a video compositor. The videocompositor receives video and graphics data from the video displaypipeline and the graphics display pipeline, respectively, and outputs tothe video encoder (“VEC”).

The system may employ a method of compositing a plurality of graphicsimages and video, which includes blending the plurality of graphicsimages into a blended graphics image, combining a plurality of alphavalues into a plurality of composite alpha values, and blending theblended graphics image and the video using the plurality of compositealpha values.

Referring to FIG. 28, a flow diagram of a process of blending video andgraphics surfaces is illustrated. The graphics display system resets instep 902. In step 904, the video compositor blends the passthrough videoand the background color with the scaled video window, using the alphavalue which is associated with the scaled video window. The result ofthis blending operation is then blended with the output of the graphicsdisplay pipeline. The graphics output has been pre-blended in thegraphics blender in step 904 and filtered in step 906, and blendedgraphics contain the correct alpha value for multiplication by the videooutput. The output of the video blend function is multiplied by thevideo alpha which is obtained from the graphics pipeline and theresulting video and graphics pixel data stream are added together toproduce the final blended result.

In general, during blending of different layers of graphics and/orvideo, every layer {L1, L2, L3 . . . Ln}, where L1 is the back-mostlayer, each layer is blended with the composition of all of the layersbehind it, beginning with L2 being blended on top of L1. Theintermediate result R(i) from the blending of pixels P(i) of layer L(i)over the pixels P(i−1) of layer L(i−1) using alpha value A(i) is:R(i)⁼A(i)*P(i)+(1−A(i))*P(i−1).

The alpha values {A(i)} are in general different for every layer and forevery pixel of every layer. However, in some important applications, itis not practical to apply this formula directly, since some layers mayneed to be processed in spatial dimensions (e.g. 2 dimensional filteringor scaling) before they can be blended with the layer or layers behindthem. While it is generally possible to blend the layers first and thenperform the spatial processing, that would result in processing thelayers that should not be processed if these layers are behind thesubject layer that is to be processed. Processing of the layers that arenot to be processed may be undesirable.

Processing the subject layer first would generally require a substantialamount of local storage of the pixels in the subject layer, which may beprohibitively expensive. This problem is significantly exacerbated whenthere are multiple layers to be processed in front of one or more layersthat are not to be processed. In order to implement the formula abovedirectly, each of the layers would have to be processed first, i.e.using their own local storage and individual processing, before theycould be blended with the layer behind.

In the preferred embodiment, rather than blending all the layers fromback to front, all of the layers that are to be processed (e.g.filtered) are layered together first, even if there is one or morelayers behind them over which they should be blended, and the combinedupper layers are then blended with the other layers that are not to beprocessed. For example, layers {1, 2 and 3} may be layers that are notto be processed, while layers {4, 5, 6, 7, and 8} may be layers that areto undergo processing, while all 8 layers are to be blended together,using {A(i)} values that are independent for every layer and pixel. Thelayers that are to be filtered, upper layers, may be the graphicswindows. The lower layers may include the video window and passthroughvideo.

In the preferred embodiment, all of the layers that are to be filtered(referred to as “upper” layers) are blended together from back to frontusing a partial blending operation. In an alternate embodiment, two ormore of the upper layers may be blended together in parallel. Theback-most of the upper layers is not in general the back-most layer ofthe entire operation.

In the preferred embodiment, at each stage of the blending, anintermediate alpha value is maintained for later use for blending withthe layers that are not to be filtered (referred to as the “lower”layers).

The formula that represents the preferred blending scheme is:R(i)=A(i)*P(i)+(1−A(i))*P(i−1)andAR(i)=AR(i−1)*(1−A(i))where R(i) represents the color value of the resulting blended pixel,P(i) represents the color value of the current pixel, A(i) representsthe alpha value of the current pixel, P(i−1) represents the value at thelocation of the current pixel of the composition of all of the upperlayers behind the current pixel, initially this represents black beforeany layers are blended, AR(i) is the alpha value resulting from eachinstance of this operation, and AR(i−1) represents the intermediatealpha value at the location of the current pixel determined from all ofthe upper layers behind the current pixel, initially this representstransparency before any layers are blended. AR represents the alphavalue that will subsequently be multiplied by the lower layers asindicated below, and so an AR value of 1 (assuming alpha ranges from 0to 1) indicates that the current pixel is transparent and the lowerlayers will be fully visible when multiplied by 1.

In other words, in the preferred. embodiment, at each stage of blendingthe upper layers, the pixels of the current layer are blended using thecurrent alpha value, and also an intermediate alpha value is calculatedas the product (1−A(i))*(AR(i−1)). The key differences between this andthe direct evaluation of the conventional formula are: (1) thecalculation of the product of the set of {(1−A(i))} for the upperlayers, and (2) a virtual transparent black layer is used to initializethe process for blending the upper layers, since the lower layers thatwould normally be blended with the upper layers are not used at thispoint in this process.

The calculation of the product of the sets of {(1−A(i)} for the upperlayers is implemented, in the preferred embodiment, by repeatedlycalculating AR(i)=AR(i−1)*(1−A(i)) at each layer, such that when alllayers {i} have been processed, the result is that AR=the product of all(1−A(i)) values for all upper layers. Alternatively in otherembodiments, the composite alpha value for each pixel of blendedgraphics may be calculated directly as the product of all (1-alpha valueof the corresponding pixel of the graphics image on each layer)'swithout generating an intermediate alpha at each stage.

To complete the blending process of the entire series of layers,including the upper and lower layers, once the upper layers have beenblended together as described above, they may be processed as. desiredand then the result of this processing, a composite intermediate image,is blended with the lower layer or layers. In addition, the resultingalpha values preferably are also processed in essentially the same wayas the image components. The lower layers can be blended in theconventional fashion, so at some point there can be a single imagerepresenting the lower layers. Therefore two images, one representingthe upper layers and one representing the lower layers can be blendedtogether. In this operation, the AR(n) value at each pixel that resultsfrom the blending of the upper layers and any subsequent processing isused to be multiplied with the composite lower layer.

Mathematically this latter operation is as follows: let L(u) be thecomposite upper layer resulting from the process described above andafter any processing, let AR(u) be the composite alpha value of theupper layers resulting from the process above and after any processing,let L(l) be the composite lower layer that results from blending alllower layers in the conventional fashion and after any processing, andlet Result be the final result of blending all the upper and lowerlayers, after any processing. Then, Result=L(u)+AR(u)*L(l). L(u) doesnot need to be multiplied by any additional alpha values, since all suchmultiplication operations were already performed at an earlier stage.

In the preferred embodiment, a series of images makes up the upperlayers. These are created by reading pixels from memory, as in aconventional graphics display device. Each pixel is converted into acommon format if it is not already in that format; in this example theYUV format is used. Each pixel also has an alpha value associated withit. The alpha values can come from a variety of sources, including (1)being part of the pixel value read from memory (2) an element in a colorlook-up table (CLUT) in cases where the pixel format uses a CLUT (3)calculated from the pixel color value, e.g. alpha as a function of Y, (4) calculated using a keying function, i.e. some pixel values aretransparent (i.e. alpha=0) and others are opaque (alpha=1) based on acomparison of the pixel value with a set of reference values, (5) analpha value may be associated with a region of the image as describedexternally, such as a rectangular region, described by the four cornersof the rectangle, may have a single alpha value associated with it, or(6) some combination of these.

The upper layers are preferably composited in memory storage bufferscalled line buffers. Each line buffer preferably is sized to containpixels of one scan line. Each line buffer has an element for each pixelon a line, and each pixel in the line buffer has elements for the colorcomponents, in this case Y, U and V, and one for the intermediate alphavalue AR. Before compositing of each line begins, the appropriate linebuffer is initialized to represent a transparent black having alreadybeen composited into the buffer; that is, the YUV value is set to thevalue that represents black (i.e. Y⁼0, U=V=128) and the alpha value ARis set to represent (1-transparent)=(1−0)=1.

Each pixel of the current layer on the current line is combined with thevalue pre-existing in the line buffer using the formulas alreadydescribed, i.e.,R(i)=A(i)*P(i)+(1−A(i))*P(i−1)andAR(i)=AR(i−1)*(1−A(i)).In other words, the color value of the current pixel P(i) is multipliedby its alpha value A(i), and the pixel in the line buffer representing.the same location on the line P(i−1) is read from the line buffer,multiplied by (1−A(i)), and added to the previous result, producing theresulting pixel value R(i). Also, the alpha value at the same locationin the line buffer (AR(i−11)) is read from the buffer and multiplied by(1−A(i)), producing AR(i). The results R(i) and AR(i) are then writtenback to the line buffer in the same location.

When multiplying a YUV value by an alpha value between 0 and 1, theoffset nature of the U and V values should preferably be accounted for.In other words, U=V=128 represents a lack of color and it is the valuethat should result from a YUV color value being multiplied by 0. Thiscan be done in at least two ways. In one embodiment of the presentinvention, 128 is subtracted from the U and V values before multiplyingby alpha, and then 128 is added to the result. In another embodiment, Uand V values are directly multiplied by alpha, and it is ensured that atthe end of the entire compositing process all of the coefficientsmultiplied by U and V sum to 1, so that the offset 128 value is notdistorted significantly.

Each of the layers in the group of upper layers is preferably compositedinto a line buffer starting with the back-most of the upper layers andprogressing towards the front until the front-most of the upper layershas been composited into the line buffer. In this way, a single hardwareblock, i.e., the display engine, may be used to implement the formulaabove for all of the upper layers. In this arrangement, the graphicscompositor engine preferably operates at a clock frequency that issubstantially higher than the pixel display rate. In one embodiment ofthe present invention, the graphics compositor engine operates at 81 MHzwhile the pixel display rate is 13.5 MHz.

This process repeats for all of the lines in the entire image, startingat the top scan line and progressing to the bottom. Once the compositingof each scan line into a line buffer has been completed, the scan linebecomes available for use in processing such as filtering or scaling.Such processing may be performed while subsequent scan lines are beingcomposited into other line buffers. Various processing operations may beselected such as anti-flutter filtering and vertical scaling.

In alternative embodiments more than one graphics layer may becomposited simultaneously, and in some such embodiments it is notnecessary to use line buffers as part of the compositing process. If allupper layers are composited simultaneously, the combination of all upperlayers can be available immediately without the use of intermediatestorage.

Referring to FIG. 29, a flow diagram of a process of blending graphicswindows is illustrated. The system preferably resets in step 920. Instep 922, the system preferably checks for a vertical sync (VSYNC). If aVSYNC has been received, the system in step 924 preferably loads a linefrom the bottom most graphics window into a graphics line buffer. Thenthe system in step 926 preferably blends a line from the next graphicswindow into the line buffer. Then the system in step 928 preferablydetermines if the last graphics window visible on a current display linehas been blended. If the last graphics window has not been blended, thesystem continues on with the blending system in step 926.

If the last window of the current display line has been reached, thesystem preferably checks in step 930 to determine if the last graphicsline of a current display field has been blended. If the last graphicsline has been blended, the system awaits another VSYNC in step 922. Ifthe last graphics line has not been blended, the system goes to the nextdisplay line in step 932 and repeats the blending process.

Referring to FIG. 30, a flow diagram of a process of receiving blendedgraphics 950, a video window 952 and a passthrough video 954 andblending them. A background color preferably is also blended in oneembodiment of the present invention. As step 956 indicates, the videocompositor preferably displays each pixel as they are composited withoutsaving pixels to a frame buffer or other memory.

When the video signals and graphics data are blended in the videocompositor, the system in step 958 preferably displays the passthroughvideo 954 outside the active window area first. There are 525 scan linesin each frame and 858 pixels in each scan line of NTSC standardtelevision signals, when a sample rate of 13.5 MHz is used, per ITU-RBt.601. An active window area of the NTSC standard television is insidean NTSC frame. There are 625 scan lines per frame and 864 pixels in eachscan line of PAL standard television, when using the ITU-R Bt.601standard sample rate of 13.5 MHz. An active window area of the PALstandard television is inside a PAL frame.

Within the active window area, the system in step 960 preferably blendsthe background color first. On top of the background color, the systemin step 962 preferably blends the portion of the passthrough video thatfalls within the active window area. On top of the passthrough window,the system in step 964 preferably blends the video window. Finally, thesystem in step 968 blends the graphics window_on top of the compositedvideo window and outputs composited video 970 for display.

Interlaced displays, such as televisions, have an inherent tendency todisplay an apparent vertical motion at the horizontal edges of displayedobjects, with horizontal lines, and on other points on the display wherethere is a sharp contrast gradient along the vertical axis. Thisapparent vertical motion is variously referred to as flutter, flicker,or judder.

While some image elements can be designed specifically for display oninterlaced TVs or filtered before they are displayed, when multiple suchimage objects are combined onto one screen, there are still visibleflutter artifacts at the horizontal top and bottom edges of theseobjects. While it is also possible to include filters in hardware tominimize visible flutter of the display, such filters are costly in thatthey require higher memory bandwidth from the display memory, since botheven and odd fields should preferably be read from memory for everydisplay field, and they tend to require additional logic and memoryon-chip.

One embodiment of the present invention includes a method of reducinginterlace flutter via automatic blending. This method has been designedfor use in graphics displays device that composites visible objectsdirectly onto the screen; for example, the device may use windows,window descriptors and window descriptor lists, or similar mechanisms.The top and bottom edges (first and last scan lines) of each object (orwindow) are displayed such that the alpha blend value (alpha blendfactor) of these edges is adjusted to be one-half of what it would be ifthese same lines were not the top and bottom lines of the window.

For example, a window may constitute a rectangular shape, and the windowmay be opaque, i.e. it's alpha blend factor is 1, on a scale of 0 to 1.All lines on this window except the first and last are opaque when thewindow is rendered. The top and bottom lines are adjusted so that, inthis case, the alpha blend value becomes 0.5, thereby causing theselines to be mixed 50% with the images that are behind them. Thisfunction occurs automatically in the preferred implementation. Since inthe preferred implementation, windows are rectangular objects that arerendered directly onto the screen, the locations of the top and bottomlines of every window are already known.

In one embodiment, the function of dividing the alpha blend values forthe top and bottom lines by two is implemented only for the top fieldsof the interlaced display. In another embodiment, the function ofdividing the alpha blend values for the top and bottom lines by two isimplemented only for the bottom fields of the interlaced display.

In the preferred embodiment, there exists also the ability to alphablend each window with the windows behind it, and this alpha value canbe adjusted for every pixel, and therefore for every scan line. Thesecharacteristics of the application design are used advantageously, asthe flutter reduction effect is implemented by controlling the alphablend function using information that is readily available from thewindow control logic.

In a specific illustrative example, the window is solid opaque white,and the image behind it is solid opaque black. In the absence of thedisclosed method, at the top and bottom edges of the window there wouldbe a sharp contrast between black and white, and when displayed on aninterlaced TV, significant flutter would be visible. Using the disclosedmethod, the top and bottom lines are blended 50% with. the background,resulting in a color that is halfway between black and white, or gray.When displayed on an interlaced TV, the apparent visual location of thetop and bottom edges of the object is constant, and flutter is notapparent. The same effect applies equally well for other image examples.

The method of reducing interlace flutter of this embodiment does notrequire any increase in memory bandwidth, as the alternate field (theone not currently being displayed) is not read from memory, and there isno need for vertical filtering, which would have required logic andon-chip memory.

The same function can alternatively be implemented in different graphicshardware designs. For example in designs using a frame buffer(conventional design), graphic objects can be composited into the framebuffer with an alpha blend value that is adjusted to one-half of itsnormal value at the top and bottom edges of each object. Such blendingcan be performed in software or in a blitter that has a blendingcapability.

XI. Anti-flutter Filtering/Vertical Scaling

In the preferred embodiment, the vertical filtering and anti-flutterfiltering are performed on blended graphics by one graphics filter. Onefunction of the graphics filter is low pass filtering in the verticaldimension. The low pass filtering may be performed in order to minimizethe “flutter” effect inherent in interlaced displays such astelevisions. The vertical downscaling or upscaling operation may beperformed in order to change the pixel aspect ratio from the squarepixels that are normal for computer, Internet and World Wide Web contentinto any of the various oblong aspect ratios that are standard fortelevisions as specified in ITU-R 601B. In order to be able to performvertical scaling of the upper layers the system preferably includesseven line buffers. This allows for four line buffers to be used forfiltering and scaling, two are available for progressing by one or twolines at the end of every line, and one for the current compositingoperation.

When scaling or filtering are performed, the alpha values in the linebuffers are filtered or scaled in the same way as the YUV values,ensuring that the resulting alpha values correctly represent the desiredalpha values at the proper location. Either or both of these operations,or neither, or other processing, may be performed on the contents of theline buffers.

Once the optional processing of the contents of the line buffers hasbeen completed, the result is the completed set of upper layers with theassociated alpha value (product of (1−A(i)). These results are useddirectly for compositing the upper layers with the lower layers, usingthe formula: Result=L(u)−AR(u).*L(1) as explained in detail in referenceto blending of graphics and video. If the lower layers require anyprocessing independent of processing required for the upper layers orfor the resulting image, the lower layers are processed before beingcombined with the upper layers; however in one embodiment of the presentinvention, no such processing is required.

Each of the operations described above is preferably implementeddigitally using conventional ASIC technology. As part of the normal ASICtechnology the logical operations are segmented into pipeline stages,which may require temporary storage of logic values from one clock cycleto the next. The choice of how many pipeline stages are used in each ofthe operations described above is dependent on the specific ASICtechnology used, the clock speed chosen, the design tools used, and thepreference of the designer, and may vary without loss of generality. Inthe preferred embodiment the line buffers are implemented as dual portmemories allowing one read and one write cycle to occur simultaneously,facilitating the read and write operations described above whilemaintaining a clock frequency of 81 MHz. In this embodiment thecompositing function is divided into multiple pipeline stages, andtherefore the address being read from the memory is different from theaddress being written to the same memory during the same clock cycle.

Each of the arithmetic operations described above in the preferredembodiment use 8 bit accuracy for each operand; this is generallysufficient for providing an accurate final result. Products are roundedto 8 bits before the result is used in subsequent additions.

Referring to FIG. 31, a block diagram illustrates an interaction betweenthe line buffers 504 and a graphics filter 172. The line bufferscomprises a set of line buffers 1-7 506 a-g. The line buffers arecontrolled by a graphics line buffer controller; over a line buffercontrol interface 502. In one embodiment of the present invention, thegraphics filter is a four-tap polyphase filter, so that four lines ofgraphics data 516 a-d are provided to the graphics filter at a time. Thegraphics filter 172 sends a line buffer release signal 516 e to the linebuffers to notify that one to three line buffers are available forcompositing additional graphics display lines.

In another embodiment, line buffers are not used, but rather all of theupper layers are composited concurrently. In this case, there is onegraphics blender for each of the upper layers active at any one pixel,and the clock rate of the graphics blender may be approximately equal tothe pixel display rate. The clock rate of the graphics blenders may besomewhat slower or faster, if FIFO buffers are used at the output of thegraphics blenders.

The mathematical formulas implemented are the same as in the firstembodiment described. The major difference is that instead of performingthe compositing function iteratively by reading and writing a linebuffer, all layers are composited concurrently and the result of theseries of compositor blocks is immediately available for processing, ifrequired, and for blending with the lower layers, and line buffers arenot used for purposes of compositing.

Line buffers may still be needed in order to implement verticalfiltering or vertical scaling, as those operations typically requiremore than one line of the group of upper layers to be availablesimultaneously, although fewer line buffers are generally required herethan in the preferred embodiment. Using multiple graphics blendersoperating at approximately the pixel rate simplifies the implementationin applications where the pixel rate is relatively fast for the ASICtechnology used, for example in HDTV video and graphics systems wherethe pixel rate is 74.25 MHz.

XII. Unified Memory Architecture/Real Time Scheduling

Recently, improvements to memory fabrication technologies have resultedin denser memory chips. However memory chip bandwidth has not beenincreasing as rapidly. The bandwidth of a memory chip is a measure ofhow fast contents of the memory chip can be accessed for reading orwriting. As a result of increased memory density without necessarily acommensurate increase in bandwidth, in many conventional system designsmultiple memory devices are used for different functions, and memoryspace in some memory modules may go unused or is wasted. In thepreferred embodiment, a unified memory architecture is used. In theunified memory architecture, all the tasks (also referred to as“clients”), including CPU, display engine and IO devices, share the samememory.

The unified memory architecture preferably includes a memory that isshared by a plurality of devices, and a memory request arbiter coupledto the memory, wherein the memory request arbiter performs real timescheduling of memory requests from different devices having differentpriorities. The unified memory system assures real time scheduling oftasks, some of which do not inherently have pre-determined periodicbehavior and provides access to memory by requesters that are sensitiveto latency and do not have determinable periodic behavior.

In an alternate embodiment, two memory controllers are used in a dualmemory controller system. The memory controllers may be 16-bit memorycontrollers or 32-bit memory controllers. Each memory controller cansupport different configuration of SDRAM device types and banks, orother forms of memory besides SDRAM. A first memory space addressed by afirst memory controller is preferably adjacent and contiguous to asecond memory space addressed by a second memory controller so thatsoftware applications view the first and second memory spaces as onecontinuous memory space. The first and the second memory controllers maybe accessed concurrently by different clients. The software applicationsmay be optimized to improve performance.

For example, a graphics memory may be allocated through the first memorycontroller while a CPU memory is allocated through the second memorycontroller. While a display engine is accessing the first memorycontroller, a CPU may access the second memory controller at the sametime. Therefore, a memory access latency of the CPU is not adverselyaffected in this instance by memory being accessed by the display engineand vice versa. In this example, the CPU may also access the firstmemory controller at approximately the same time that the display engineis accessing the first memory controller, and the display controller canaccess memory from the second memory controller, thereby allowingsharing of memory across different functions, and avoiding many copyoperations that may otherwise be required in conventional designs.

Referring to FIG. 32, a dual memory controller system services memoryrequests generated by a display engine 1118, a CPU 1120, a graphicsaccelerator 1124 and an input/output module 1126 are provided to amemory select block 1100. The memory select block 1100 preferably routesthe memory requests to a first arbiter 1102 or to a second arbiter 1106based on the address of the requested memory. The first arbiter 1102sends memory requests to a first memory controller 1104 while the secondarbiter 1106 sends memory requests to a second memory controller 1108.The design of arbiters for handling requests from tasks with differentpriorities is well known in the art.

The first memory controller preferably sends address and control signalsto a first external SDRAM and receives a first data from the firstexternal SDRAM. The second memory controller preferably sends addressand control signals to a second external SDRAM and receives a seconddata from the second external SDRAM.

The first and second memory controllers preferably provide first andsecond data received, respectively, from the first and second externalSDRAMs to a device that requested the received data.

The first and second data from the first and second memory controllersare preferably multiplexed, respectively, by a first multiplexer 1110 atan input of the display engine, by a second multiplexer 1112 at an inputof the CPU, by a third multiplexer 1114 at an input of the graphicsaccelerator and by a fourth multiplexer 1116 at an input of the I/Omodule. The multiplexers provide either the first or the second data, asselected by memory select signals provided by the memory select block,to a corresponding device that has requested memory.

An arbiter preferably uses an improved form of real time scheduling tomeet real-time latency requirements while improving performance forlatency-sensitive tasks. First and second arbiters may be used with theflexible real time scheduling. The real time scheduling is preferablyimplemented on both the first arbiter and the second arbiterindependently.

When using a unified memory, memory latencies caused by competing memoryrequests by different tasks should preferably be addressed. In thepreferred embodiment, a real-time scheduling and. arbitration scheme forunified memory is implemented, such that all tasks that use the unifiedmemory meet their real-time requirements. With this innovative use ofthe unified memory architecture and real-time scheduling, a singleunified memory is provided to the CPU and other devices of the graphicsdisplay system without compromising quality of graphics or otheroperations and while simultaneously minimizing the latency experiencedby the CPU.

The methodology used preferably implements real-time scheduling usingRate Monotonic Scheduling (“RMS”). It is a mathematical approach thatallows the construction of provably correct schedules of arbitrarynumbers of real-time tasks with arbitrary periods for each of the tasks.This methodology provides for a straight forward means for proof bysimulation of the worst case scenario, and this simulation is simpleenough that it can be done by hand. RMS, as normally applied, makes anumber of simplifying assumptions in the creation of a priority list.

In the normal RMS assumptions, all tasks are assumed to have constantperiods, such that a request for service is made by the task with statedperiod, and all tasks have a latency tolerance that equals that task'speriod. Latency tolerance is defined as the maximum amount of time thatcan pass from the moment the task requests service until that task'srequest has been completely satisfied. During implementation of oneembodiment of the present invention, the above assumptions have beenmodified, as described below.

In the RMS method, all tasks are generally listed along with theirperiods. They are then ordered by period, from the shortest to thelongest, and priorities are assigned in that order. Multiple tasks withidentical periods can be in any relative order. In other words, therelative order amongst them can be decided by, for example, flipping acoin.

Proof of correctness, i.e. the guarantee that all tasks meet theirdeadlines, is constructed by analyzing the behavior of the system whenall tasks request service at exactly the same time; this time is calledthe “critical instant”. This is the worst case scenario, which may notoccur in even a very large set of simulations of normal operation, orperhaps it may never occur in normal operation, however it is presumedto be possible. As each task is serviced, it uses the shared resource,memory clock cycles in the present invention, in the degree stated bythat task. If all tasks meet their deadlines, the system is guaranteedto meet all tasks' deadlines under all conditions, since the criticalinstant analysis simulates the worst case.

When the lowest priority real-time task meets its deadline, without anyhigher priority tasks missing their deadlines, then all tasks are provento meet their deadlines. As soon as any task in this simulation fails tomeet its deadline, the test has failed and the task set cannot beguaranteed, and therefore the design should preferably be changed inorder to guarantee proper operation under worst case conditions.

In the RMS methodology, real-time tasks are assumed to have periodicrequests, and the period and the latency tolerance are assumed to havethe same value. Since the requests may not be in fact periodic, it isclearer to speak in terms of “minimum interval” rather than period. Thatis, any task is assumed to be guaranteed not to make two consecutiverequests with an interval between them that is any shorter than theminimum interval.

The deadline, or the latency tolerance, is the maximum amount of timethat may pass between the moment a task makes a request for service andthe time that the service is completed, without impairing the functionof the task. For example, in a data path with a constant rate source (orsink), a FIFO, and memory access from the FIFO, the request may occur assoon as there is enough data in the FIFO that if service is grantedimmediately the FIFO does not underflow (or overflow in case of a readoperation supporting a data sink) . If service is not completed beforethe FIFO overflows (or underflows in the case of a data sink) the taskis impaired.

In the RMS methodology, those tasks that do not have specified real-timeconstraints are preferably grouped together and served with a singlemaster task called the “sporadic server”, which itself has the lowestpriority in the system. Arbitration within the set of tasks served bythe sporadic server is not addressed by the RMS methodology, since it isnot a real-time matter. Thus, all non-real-time tasks are servedwhenever there is resource available, however the latency of serving anyone of them is not guaranteed.

To implement real-time scheduling based on the RMS methodology, first,all of the tasks or clients that need to access memory are preferablylisted, not necessarily in any particular order. Next, the period ofeach of the tasks is preferably determined. For those with specificbandwidth requirements (in bytes per second of memory access), theperiod is preferably calculated from the bandwidth and the burst size.If the deadline is different from the period for any given task, that islisted as well. The resource requirement when a task is serviced islisted along with the task. In this case, the resource requirement isthe number of memory clock cycles required to service the memory accessrequest. The tasks are sorted in order of increasing period, and theresult is the set of priorities, from highest to lowest. If there aremultiple tasks with the same period, they can be given different,adjacent priorities in any random relative order within the group; orthey can be grouped together and served with a single priority, withround-robin arbitration between those tasks at the same priority.

In practice, the tasks sharing the unified memory do not all have trueperiodic behavior. In one embodiment of the present invention, a blockout timer, associated with a task that does not normally have a period,is used in order to force a bounded minimum interval, similar to aperiod, on that task. For example a block out timer associated with theCPU has been implemented in this embodiment. If left uncontrolled, theCPU can occupy all available memory cycles, for example by causing anever-ending stream of cache misses and memory requests. At the sametime, CPU performance is determined largely by “average latency ofmemory access”, and so the CPU performance would be less than optimal ifall CPU memory accessed were consigned to a sporadic server, i.e., atthe lowest priority.

In this embodiment, the CPU task has been converted into two logicaltasks. A first CPU task has a very high priority for low latency, and italso has a block out timer associated with it such that once a requestby the CPU is made, it cannot submit a request again until the block outtimer has timed out. In this embodiment, the CPU task has the toppriority. In other embodiments, the CPU task may have a very highpriority but not the top priority. The timer period has been madeprogrammable for system tuning, in order to accommodate different systemconfigurations with different memory widths or other options.

In one embodiment of the present invention, the block out timer isstarted when the CPU makes a high priority request. In anotherembodiment, the block out timer is started when the high priorityrequest by the CPU is serviced. In other embodiments, the block outtimer may be started at any time in the interval between the time thehigh priority request is made and the time the high priority request isserviced.

A second CPU task is preferably serviced by a sporadic server in around-robin manner. Therefore if the CPU makes a long string of memoryrequests, the first one is served as a high priority task, andsubsequent requests are served by the low priority sporadic serverwhenever none of the real-time tasks have requests pending, until theCPU block out timer times out. In one embodiment of the presentinvention, the graphics accelerator and the display engine are alsocapable of requesting more memory cycles than are available, and so theytoo use similar block out timer.

For example, the CPU read and write functions are grouped together andtreated as two tasks. A first task has a theoretical latency bound of 0and a period that is programmable via a block out timer, as describedabove. A second task is considered to have no period and no deadline,and it is grouped into the set of tasks served by the sporadic servervia a round robin at the lowest priority. The CPU uses a programmableblock out timer between high priority requests in this embodiment.

For another example, a graphics display task is considered to have aconstant bandwidth of 27 MB/s, i.e., 16 bits per pixel at 13.5 MHz.However, the graphics bandwidth in one embodiment of the presentinvention can vary widely from much less than 27 MB/s to a much greaterfigure, but 27 MB/s is a reasonable figure for assuring support of arange of applications. For example, in one embodiment of the presentinvention, the graphics display task utilizes a block out timer thatenforces a period of 2.37 μs between high priority requests, whileadditional requests are serviced on a best-effort basis by the sporadicserver in a low priority round robin manner.

Referring to FIG. 33, a block diagram illustrates an implementation of areal-time scheduling using an RMS methodology. A CPU service request1138 is preferably coupled to an input of a block out timer 1130 and asporadic server 1136. An output of the block out timer 1130 ispreferably coupled to an arbiter 1132 as a high priority servicerequest. Tasks 1-5 1134 a-e may also be coupled to the arbiter asinputs. An output of the arbiter is a request for service of a task thathas the highest priority among all tasks that have a pending memoryrequest.

In FIG. 33, only the CPU service request 1138 is coupled to a block outtimer. In other embodiments, service. requests from other tasks may becoupled to their respective block out timers. The block out timers areused to-enforce a minimum interval between two successive accesses byany high priority task that is non-periodic but may require expeditedservicing. Two or more such high priority tasks may be coupled to theirrespective block out timers in one embodiment of the present invention.Devices that are coupled to their respective block out timers as highpriority tasks may include a graphics accelerator, a display engine, andother devices.

In addition to the CPU request 1138, low priority tasks 1140 a-d may becoupled to the sporadic server 1136. In the sporadic server, these lowpriority tasks are handled in a round robin manner. The sporadic serversends a memory request 1142 to the arbiter for the next low prioritytask to be serviced.

Referring to FIG. 34, a timing diagram illustrates CPU service requestsand services in case of a continuous CPU request 1146. In practice, theCPU request is generally not continuous, but FIG. 34 has been providedfor illustrative purposes. In the example represented in FIG. 34, ablock out timer 1148 is started upon a high priority service request1149 by the CPU. At time t₀, the CPU starts making the continuousservice request 1146, and a high priority service request 1149 is firstmade provided that the block out timer 1148 is not running at time t₀.When the high priority service request is made, the block out timer 1148is started. Between time t₀and time t₁, the memory controller finishesservicing a memory request from another task. The CPU is first servicedat time t₁. In the preferred embodiment, the duration of the block outtimer is programmable. For example, the duration of the block out timermay be programmed to be 3 μs.

Any additional high priority CPU request 1149 is blocked out until theblock out timer times out at time t₂. Instead, the CPU low priorityrequest 1150 is handled by a sporadic server. in a round robin mannerbetween time t₀ and time t₂. The low priority request 1150 is active aslong as the CPU service request is active. Since the CPU service request1146 is continuous, another high priority service request 1149 is madeby the CPU and the block out timer is started again as soon as the blockout timer times out at time t₂. The high priority service request madeby the CPU at time t₂ is serviced at time t₃ when the memory controllerfinishes servicing another task. Until the block out timer times out attime t₄, the CPU low priority request 1150 is handled by the sporadicserver while the CPU high priority request 1149 is blocked out.

Another high priority service request is made and the block out timer1148 is started again when the block out timer 1148 times out at timet₄. At time t₅, the high priority service request 1149 made by the CPUat time t₄ is serviced. The block out timer does not time out until timet₇. However, the block out timer is not in the path of the CPU lowpriority service request and, therefore, does not block out the CPU lowpriority service request. Thus, while the block out timer is stillrunning, a low priority service request made by the CPU is handled bythe sporadic server, and serviced at time t₆.

When the block out timer 1148 times out at time t₇, it is started againand yet another high priority service request is made by the CPU, sincethe CPU service request is continuous. The high priority service request1149 made by the CPU at time t₇ is serviced at time t₈. When the blockout timer times out at time t₉, the high priority service request isonce again made by the CPU and the block out timer is started again.

The schedule that results from the task set and priorities above isverified by simulating the system performance starting from the“critical instant”, when all tasks request service at the same time anda previously started low priority task is already underway. The systemis proven to meet all the real-time deadlines if all of the tasks withreal-time deadlines meet their deadlines. Of course, in order to performthis simulation accurately, all tasks make new requests at everyrepetition of their periods, whether or not previous requests have beensatisfied.

Referring to FIG. 35, a timing diagram illustrates an example of acritical instant analysis. At time t₀, a task 1 1156, a task 2 1158, atask 3 1160 and a task 4 1162 request service at the same time. Further,at time t₀, a low priority task 1154 is being serviced. Therefore, thehighest priority task, the task 1, cannot be serviced until servicing ofthe low priority task has been completed.

When the low priority task is completed at time t₁, the task 1 isserviced. Upon completion of the task 1 at time t₂, the task 2 isserviced. Upon completion of the task 2 at time t₃, the task 3 isserviced. Upon completion of the task 3 at time t₄, the task 4 isserviced. The task 4 completes at time t₅, which is before the start ofa next set of tasks: the task 1 at t₆, the task 2 at t₇, the task 3 att₈, and the task 4 at t₉.

For example, referring to FIG. 36, a flow diagram illustrates a processof servicing memory requests with different priorities, from the highestto the lowest. The system in step 1170 makes a CPU read request with thehighest priority. Since a block out timer is used with the CPU readrequest in this example, the block out timer is started upon making thehighest priority CPU read request. Then the system in step 1172 makes agraphics read request. A block out timer is also used with the graphicsread request, and the block out timer is started upon making thegraphics read request.

A video window read request in step 1174 and a video capture writerequest in step 1176 have equal priorities. Therefore, the video windowread request and the video capture write request are placed in a roundrobin arbitration for two tasks (clients). The system in step 1178 andstep 1180 services a refresh request and a audio read request,respectively.

While respective block out timers for the CPU read request and thegraphics read request are active, the system places the CPU read requestand the graphics read request in a round robin arbitration for fivetasks (clients), respectively, in step 1182 and step 1186. The system insteps 1184, 1188 and 1190 places other lowest priority tasks such as agraphics accelerator read/write request, a DMA read/write request and aCPU write request, respectively, in this round robin arbitration withfive clients.

XIII. Graphics Accelerator

Displaying of graphics generally requires a large amount of processing.If all processing of graphics is performed by a CPU, the processingrequirements may unduly burden the CPU since the CPU generally alsoperforms many other tasks. Therefore, many systems that perform graphicsprocessing use a dedicated processor, which is typically referred to asa graphics accelerator.

The system according to the present invention may employ a graphicsaccelerator that includes memory for graphics data, the graphics dataincluding pixels, and a coprocessor for performing vector typeoperations on a plurality of components of one pixel of the graphicsdata.

The preferred embodiment of the graphics display system uses a graphicsaccelerator that is optimized for performing real-time 3D and 2D effectson graphics and video surfaces. The graphics accelerator preferablyincorporates specialized graphics vector arithmetic functions formaximum performance with video and real-time graphics. The graphicsaccelerator performs a range of essential graphics and video operationswith performance comparable to hardwired approaches, yet it isprogrammable so that it can meet new and evolving applicationrequirements with firmware downloads in the field. The graphicsaccelerator is preferably capable of 3D effects such as real-time videowarping and flipping, texture mapping, and Gouraud and Phong polygonshading, as well as 2D and image effects such as blending, scaling,blitting and filling. The graphics accelerator and its caches arepreferably completely contained in an integrated circuit chip.

The graphics accelerator of the present invention is preferably based ona conventional RISC-type microprocessor architecture. The graphicsaccelerator preferably also includes additional features and somespecial instructions in the instruction set. In the preferredembodiment, the graphics accelerator is based on a MIPS R3000 classprocessor. In other embodiments, the graphics accelerator may be basedon almost any other type of processors.

Referring to FIG. 37, a graphics accelerator 64 receives commands from aCPU 22 and receives graphics data from main memory 28 through a memorycontroller 54. The graphics accelerator preferably includes acoprocessor (vector coprocessor) 1300 that performs vector typeoperations on pixels. In vector type operations, the R, G, and Bcomponents, or the Y, U and V components, of a pixel are processed inparallel as the three elements of a “vector”. In alternate embodiments,the graphics accelerator may not include the vector coprocessor, and thevector coprocessor may be coupled to the graphics accelerator instead.The vector coprocessor 1300 obtains pixels (3-tuple vectors) via aspecialized LOAD instruction.

The LOAD instruction preferably extracts bits from a 32-bit word inmemory that contains the required bits. The LOAD instruction alsopreferably packages and converts the bits into the input vector formatof the coprocessor. The vector coprocessor 1300 writes pixels (3-tuplevectors) to memory via a specialized STORE instruction. The STOREinstruction preferably extracts the required bits from the accumulator(output) register of the coprocessor, converts them if required, andpacks them into a 32-bit word in memory in a format suitable for otheruses within the IC, as explained below.

Formats of the 32-bit word in memory preferably include an RGB16 formatand a YUV format. When the pixels are formatted in RGB16 format, R has 5bits, G has 6 bits, and B has 5 bits. Thus, there are 16 bits in eachRGB16 pixel and there are two RGB16 half-words in every 32-bit word inmemory. The two RGB16 half-words are selected, respectively, viaVectorLoadRGB16Left instruction and VectorLoadRGB16Right instruction.The 5 or 6 bit elements are expanded through zero expansion into 8 bitcomponents when loaded into the coprocessor input register 1308.

The YUV format preferably includes YUV 4:2:2 format, which has fourbytes representing two pixels packed into every 32-bit word in memory.The U and V elements preferably are shared between the two pixels. Atypical packing format used to load two pixels having YUV 4:2:2 formatinto a 32-bit memory is YUYV, where each of first and second Y's, U andV has eight bits. The left pixel is preferably comprised of the first Yplus the U and V, and the right pixel is preferably comprised of thesecond Y plus the U and V. Special LOAD instructions, LoadYUVLeft andLoadYUVRight, are preferably used to extract the YUV values for the leftpixel and the right pixel, respectively, and put them in the coprocessorinput register 1308.

Special STORE instructions, StoreVectorAccumulatorRGB16,StoreVectorAccumulatorRGB24, StoreVectorAccumulatorYUVLeft, andStoreVectorAccumulatorYUVRight, preferably convert the contents of theaccumulator, otherwise referred to as the output register of thecoprocessor, into a chosen format for storage in memory. In the case ofStoreVectorAccumulatorRGB16, the three components (R, G, and B) in theaccumulator typically have 8, 10 or more significant bits each; theseare rounded or dithered to create R, G, and B values with 5, 6, and 5bits respectively, and packed into a 16 bit value. This 16 bit value isstored in memory, selecting either the appropriate 16 bit half word inmemory via the store address.

In the case of StoreVectorAccumulatorRGB24, the R, G, and B componentsin the accumulator are rounded or dithered to create 8 bit values foreach of the R, G, and B components, and these are packed into a 24 bitvalue. The 24 bit RGB value is written into memory at the memory addressindicated via the store address. In the cases ofStoreVectorAccumulatorYUVLeft and StoreVectorAccumulatorYUVRight, the Y,U and V components in the accumulator are dithered or rounded to create8 bit values for each of the components.

In the preferred embodiment, the StoreVectorAccumulatorYUVLeftinstruction writes the Y, U and V values to the locations in theaddressed memory word corresponding to the left YUV pixel, i.e. the wordis arranged as YUYV, and the first Y value and the U and V values areover-written. In the preferred embodiment, theStoreVectorAccumulatorYUVRight instruction writes the Y value to thememory location corresponding to the Y component of the right YUV pixel,i.e. the second Y value in the preceding example. In other embodimentsthe U and V values may be combined with the U and V values already inmemory creating a weighted sum of the existing and stored values andstoring the result.

The coprocessor instruction set preferably also includes aGreaterThanOREqualTo (GE) instruction. The GE instruction performs agreater-than-or-equal-to comparison between each element of a pair of3-element vectors. Each element in each of the 3 -element vectors has asize of one byte. The results of all three comparisons, one bit per eachresult, are placed in a result register 1310, which may subsequently beused for a single conditional branch operation. This saves a lot ofinstructions (clock cycles) when performing comparisons between all theelements of two pixels.

The graphics accelerator preferably includes a data SRAM 1302, alsocalled a scratch pad memory, and not a conventional data cache. In otherembodiments, the graphics accelerator may not include the data SRAM, andthe data SRAM may be coupled to the graphics accelerator instead. Thedata SRAM 1302 is similar to a cache that is managed in software. Thegraphics accelerator preferably also includes a DMA engine 1304 withqueued commands. In other embodiments, the graphics accelerator may notinclude the DMA engine, and the DMA engine may be coupled to thegraphics accelerator instead. The DMA engine 1304 is associated with thedata SRAM 1302 and preferably moves data between the data SRAM 1302 andmain memory 28 at the same time the graphics accelerator 64 is using thedata SRAM 1302 for its load and-store operations. In the preferredembodiment, the main memory 28 is the unified memory that is shared bythe graphics display system, the CPU 22, and other peripherals.

The DMA engine 1304 preferably transfers data between the memory 28 andthe data SDRAM 1302 to carry out load and store instructions. In otherembodiments, the DMA engine 1304 may transfer data between the memory 28and other components of the graphics accelerator without using the dataSRAM 1302. Using data SRAM, however, generally results in faster loadingand storing operations.

The DMA engine 1304 preferably has a queue 1306 to hold multiple DMAcommands, which are executed sequentially in the order they arereceived. In the preferred embodiment, the queue 1306 is fourinstructions deep. This may be valuable because the software (firmware)may be structured so that the loop above the inner loop may instruct theDMA engine 1304 to perform a series of transfers, e.g. to get two setsof operands and write one set of results back, and then the inner loopmay execute for a while; when the inner loop is done, the graphicsaccelerator 64 may check the command queue 1306 in the DMA engine 1304to see if all of the DMA commands have been completed. The queueincludes a mechanism that allows the graphics accelerator to determinewhen all the DMA commands have been completed. If all of the DMAcommands have been completed, the graphics accelerator 64 preferablyimmediately proceeds to do more work, such as commanding additional DMAoperations to be performed and to do processing on the new operands. Ifnot, the graphics accelerator 64 preferably waits for the completion ofDMA commands or perform some other tasks for a while.

Typically, the graphics accelerator 64 is working on operands andproducing outputs for one set of pixels, while the DMA engine 1304 isbringing in operands for the next (future) set of pixel operations, andalso the DMA engine 1304 is writing back to memory the results from theprevious set of pixel operations. In this way, the graphics accelerator64 does not ever have to wait for DMA transfers (if the code is designedwell), unlike a conventional data cache, wherein the conventional datacache gets new operands only when there is a cache miss, and it writesback results only when either the cache writes it back automaticallybecause it needs the cache line for new operands or when there is anexplicit cache line flush operation performed. Therefore, the graphicsaccelerator 64 of the present invention preferably reduces or eliminatesperiod of waiting for data, unlike conventional graphics acceleratorswhich may spend a large fraction of their time waiting for data transferoperations between the cache and main memory.

Although this invention has been described in certain specificembodiments, many additional modifications and variations would beapparent to those skilled in the art. It is therefore to be understoodthat this invention may be practiced otherwise than as specificallydescribed. Thus, the present embodiments of the invention should beconsidered in all respects as illustrative and not restrictive, thescope of the invention to be determined by the appended claim and theirequivalents.

1. A method of displaying a graphical element comprising the steps of:filtering the graphical element with a low pass filter to generate amulti-level value per pixel at an intended final display resolution;reducing a gray level of the multi-level values by at least one bit togenerate reduced multi-level values for use as alpha blend values; andusing the reduced multi-level values as the alpha blend values for thegraphical element in a subsequent compositing stage, wherein generationof the multi-level values do not depend on alpha blend values thatexisted prior to filtering.
 2. The method of displaying a graphicalelement of claim 1 wherein the reduced multi-level values are writteninto a display buffer where the reduced multi-level values are used asthe alpha blend values when contents of the display buffer arecomposited with other graphics and video images.
 3. The method ofdisplaying a graphical element of claim 1 wherein the graphical elementis initially rendered at a higher resolution than the intended finaldisplay resolution.
 4. The method of displaying a graphical element ofclaim 3 wherein the graphical element is initially rendered at fourtimes the resolution of the intended final display resolution in atleast a horizontal direction.
 5. The method of displaying a graphicalelement of claim 3 wherein the graphical element is initially renderedat four times the resolution of the intended final display resolution inat least a vertical direction.
 6. The method of displaying a graphicalelement of claim 1 wherein the low pass filter is a box filter.
 7. Amethod of displaying a graphical element comprising the steps of:filtering the graphical element with a low pass filter to generate amulti-level value per pixel at an intended final display resolution;reducing a gray level of the multi-level values by at least one bit togenerate reduced multi-level values; storing the reduced multi-levelvalues in a display buffer; and using the reduced multi-level values asalpha blend values for the graphical element in a subsequent compositingstage, wherein the graphical element includes text, and the displaybuffer is defined to have a constant foreground color that is consistentwith a desired foreground color of the text.
 8. A method of displaying agraphical element comprising the steps of: filtering the graphicalelement with a low pass filter to generate a multi-level value per pixelat an intended final display resolution; reducing a gray level of themulti-level values by at least one bit to generate reduced multi-levelvalues; storing the reduced multi-level values in a display buffer; andusing the reduced multi-level values as alpha blend values for thegraphical element in a subsequent compositing stage, wherein thegraphical element includes graphics, and the display buffer is definedto have a constant foreground color that is consistent with a desiredforeground color of the graphics.
 9. The method of displaying agraphical element of claim 1 wherein the alpha blend values include CLUTindexes, each CLUT index is associated with a CLUT entry, and each CLUTentry contains a CLUT alpha blend value.
 10. The method of displaying agraphical element of claim 1 wherein the alpha blend values are used toform alpha portions of pixels having a color portion and an alphaportion.
 11. The method of displaying a graphical element of claim 10wherein the pixels having color portions and alpha portions are in analphaRGB (4,4,4,4) format.
 12. The method of displaying a graphicalelement of claim 1 wherein the graphical element has a plurality offoreground colors, which are filtered using the low pass filter.
 13. Themethod of displaying a graphical element of claim 12 wherein thefiltered plurality of foreground colors are used as color portions ofpixels having a color portion and an alpha portion.
 14. The method ofdisplaying a graphical element of claim 13 wherein the pixels having acolor portion and an alpha portion are in an alphaRGB format.
 15. Themethod of displaying a graphical element of claim 13 wherein the pixelshaving a color portion and an alpha portion are in an alphaYUV format.16. The method of displaying a graphical element of claim 12 wherein thefiltered plurality of foreground colors are used as color choices in aCLUT format.
 17. The method of displaying a graphical element of claim 1wherein an outline of the graphical element, including all colors otherthan background color, is filtered using the low pass filter, whereinthe graphical element has a plurality of foreground colors.
 18. Themethod of displaying a graphical element of claim 17 wherein thefiltered outline is used as an alpha per pixel value.
 19. The method ofdisplaying a graphical element of claim 18 wherein the filtered outlineis used as the alpha per pixel value in a direct color format, thedirect color format including an alphaRGB format.
 20. The method ofdisplaying a graphical element of claim 18 wherein the filtered outlineis used as a choice of an alpha value per CLUT entry in a CLUT format.21. A method of displaying a graphical element comprising the steps of:filtering the graphical element with a low pass filter to generate amulti-level value per pixel at an intended final display resolution;reducing a gray level of the multi-level values by at least one bit togenerate reduced multi-level values for use as alpha blend values; andusing the reduced multi-level values as the alpha blend values for thegraphical element in a subsequent compositing stage, wherein the reducedmulti-level values are written into a display buffer where the reducedmulti-level values are used as alpha blend values when contents of thedisplay buffer are composited with other graphics and video images, andwherein the step of using the reduced multi-level values as the alphablend values for the graphical element in a subsequent compositing stagecomprises compositing the display buffer with other graphics and videocontents while blending the display buffer with all layers behind itusing alpha per pixel values.
 22. A method of displaying a graphicalelement comprising the steps of: filtering the graphical element with alow pass filter to generate a multi-level value per pixel at an intendedfinal display resolution; reducing a gray level of the multi-levelvalues by at least one bit to generate reduced multi-level values; andusing the reduced multi-level values as alpha blend values for thegraphical element in a subsequent compositing stage, wherein the reducedmulti-level values are written into a display buffer where the reducedmulti-level values are used as the alpha blend values when contents ofthe display buffer are composited with other graphics and video images,and wherein opacity of the graphical element may be varied by specifyingan alpha value of the display buffer.
 23. A graphics display system fordisplaying a graphical element comprising: a low pass filter forfiltering the graphical element to generate multi-level values, onemulti-level value per each pixel, at an intended final displayresolution; a display buffer for storing the multi-level values; and adisplay engine for compositing the graphical element with at least onegraphics image using the multi-level values as alpha blend values,wherein generation of the multi-level values do not depend on alphablend values that existed prior to filtering, and wherein a gray levelof the multi-level values is reduced by at least one bit prior to usingthem as the alpha blend values.
 24. The graphics display system fordisplaying a graphical element of claim 23 wherein the graphical elementis initially rendered at a higher resolution than the intended finaldisplay resolution.
 25. The graphics display system of claim 23 whereinthe low pass filter is a box filter.
 26. A graphics display system fordisplaying a graphical element comprising: a low pass filter forfiltering the graphical element to generate multi-level values, onemulti-level value per each pixel, at an intended final displayresolution; a display buffer for storing the multi-level values; and adisplay engine for compositing the graphical element with graphicsimages using the multi-level values as alpha blend values, wherein thegraphical element includes text, and the display buffer is defined tohave a constant foreground color that is consistent with a desiredforeground color of the text, and wherein a gray level of themulti-level values is reduced by at least one bit prior to using them asthe alpha blend values.
 27. A graphics display system for displaying agraphical element comprising: a low pass filter for filtering thegraphical element to generate multi-level values, one multi-level valueper each pixel, at an intended final display resolution; a displaybuffer for storing the multi-level values; and a display engine forcompositing the graphical element with graphics images using themulti-level values as alpha blend values, wherein the graphical elementincludes graphics, and the display buffer is defined to have a constantforeground color that is consistent with a desired foreground color ofthe graphics, and wherein a gray level of the multi-level values isreduced by at least one bit prior to using them as the alpha blendvalues.
 28. The graphics display system of claim 23 wherein the alphablend values include CLUT indexes, each CLUT index is associated with aCLUT entry, and each CLUT entry contains a CLUT alpha blend value. 29.The graphics display system of claim 23 wherein the alpha blend valuesare used to form alpha portions of pixels having a color portion and analpha portion.
 30. The graphics display system of claim 29 wherein thepixels having a color portion and an alpha portion are in an alphaRGB(4,4,4,4) format.
 31. The graphics display system of claim 23 whereinthe graphical element has a plurality of foreground colors, which arefiltered using the low pass filter.
 32. The graphics display system ofclaim 31 wherein the filtered plurality of foreground colors are used ascolor portions of pixels having a color portion and an alpha portion.33. The graphics display system of claim 32 wherein the pixels having acolor portion and an alpha portion are in an alphaRGB format.
 34. Thegraphics display system of claim 32 wherein the pixels having a colorportion and an alpha portion are in an alphaYUV format.
 35. The graphicsdisplay system of claim 31 wherein the filtered plurality of foregroundcolors are used as color choices in a CLUT format.
 36. The graphicsdisplay system of claim 23 wherein an outline of the graphical element,including all colors other than background color, is filtered using thelow pass filter, wherein the graphical element has a plurality offoreground colors.
 37. The graphics display system of claim 36 whereinthe filtered outline is used as an alpha per pixel value.
 38. Thegraphics display system of claim 37 wherein the filtered outline is usedas the alpha per pixel value in a direct color format, the direct colorformat including an alphaRGB format.
 39. A graphics display system fordisplaying a graphical element comprising: a low pass filter forfiltering the graphical element to generate multi-level values, onemulti-level value per each pixel, at an intended final displayresolution; a display buffer for storing the multi-level values; and adisplay engine for compositing the graphical element with graphicsimages using the multi-level values as alpha blend values, wherein agray level of the multi-level values is reduced by at least one bitprior to using them as the alpha blend values, wherein an outline of thegraphical element, including all colors other than background color, isfiltered using the low pass filter, wherein the graphical element has aplurality of foreground colors wherein the filtered outline is used asan alpha per pixel value, and wherein the filtered outline is used as achoice of an alpha value per CLUT entry in a CLUT format.
 40. A graphicsdisplay system for displaying a graphical element comprising: a low passfilter for filtering the graphical element to generate multi-levelvalues, one multi-level value per each pixel, at an intended finaldisplay resolution; a display buffer for storing the multi-level values;and a display engine for compositing the graphical element with graphicsimages, wherein translucency of the graphical element is varied byspecifying the alpha value of the display buffer, and wherein a graylevel of the multi-level values is reduced by at least one bit prior tousing them as the alpha blend values.